Sonnet CXXXIX

O! call not me to justify the wrong
That thy unkindness lays upon my heart;
Wound me not with thine eye, but with thy tongue:
Use power with power, and slay me not by art,
Tell me thou lov'st elsewhere; but in my sight,
Dear heart, forbear to glance thine eye aside:
What need'st thou wound with cunning, when thy might
Is more than my o'erpressed defence can bide?
Let me excuse thee: ah! my love well knows
Her pretty looks have been mine enemies;
And therefore from my face she turns my foes,
That they elsewhere might dart their injuries:
   Yet do not so; but since I am near slain,
   Kill me outright with looks, and rid my pain.

The 1609 Quarto Version

OCall not me to iuſtifie the wrong,
That thy vnkindneſſe layes vpon my heart,
Wound me not with thine eye but with thy toung,
Vſe power with power,and ſlay me not by Art,
Tell me thou lou'ſt elſe-where;but in my ſight,
Deare heart forbeare to glance thine eye aſide,
What needſt thou wound with cunning when thy                                                                                  might
Is more then my ore-preſt defence can bide?
Let me excuſe thee,ah my loue well knowes,
Her prettie lookes haue beene mine enemies,
And therefore from my face ſhe turnes my foes,
That they elſe-where might dart their iniuries :
   Yet do not ſo,but ſince I am neere ſlaine,
   Kill me out-right with lookes,and rid my paine.

Commentary

1. O! call not me to justify the wrong

O! call not me = do not insist that I etc. The language could be that of the courtroom - call the witness to the stand -, as in 35 and 49, which use legal terms (Thy adverse party is thy advocate--) to defend the beloved youth against the charge of betrayal.
to justify the wrong = to defend you against the charge of wrongdoing. What the crime is is never accurately specified, but we infer from the traditions that it is typical of the harsh cruelty which the beloved was expected to show towards the passionate lover. In the following lines it is the pain caused by the beloved's indifferent eyes, which nevertheless flirt with other men. But in addition we tend to deduce, judging by the previous five sonnets, that the unkindness is that she is not interested in him at all and sleeps with other men. When he decides later on in the sestet to defend her, (line 9), the wrong is the conventional one of wounding him with her eyes, as Laura wounded Petrarch, and as many Celias, Chlorises, Delias, Fidessas, Stellas and others wounded their patient adorers. As portrayed by the sonneteers, these beauties remained aloof from their lover's pleas, although they occasionally smiled, and even less frequently allowed a kiss. Beauty and chastity were their defining features. The difference with Shakespeare's dark lady is that she is neither beautiful (at least not in the conventional sense), and certainly not chaste. The sonnet conventions have therefore been more or less turned upside down and brought close to ridicule. Whether this element of ridicule and parody is the predominant force in these sonnets, rather than the personal anguish which springs from the experience of loving, it is impossible to say. It is partly our own unfamiliarity with this world of courtiers and sonneteers with its conventionally defined love charades that makes it so difficult for us to judge the extent to which direct human experience overcame convention and allowed what we might regard as ordinary emotion, or even love, to intrude. It is possible to love even according to formulae and rules, but it often happens that the reality of desire breaks down the conventions. Ideally the beloved is inaccessible because she is naturally chaste. She could even be married to another man, which makes her doubly fenced in, for she cannot break her vows and she does not wish to gratify stray men. That situation does not here apply, because, as the poet has depicted her, even if she is married, she still welcomes other men to enjoy her 'treasure'. The traditional background which we might therefore put in place to interpret the sonnet falls away, and we are inevitably led to read it in a much more glaring light, and to assess the relationship of loved and lover without the usual constraint of pre-conditions.

2. That thy unkindness lays upon my heart;

thy unkindness = your cruelty (of abandoning me. See the note above). Your unnaturalness, in not behaving as those of your kind (type, species) should do. The unnaturalness lay in not being like the conventional chaste beauty of most sonnets.
lays upon my heart = inflicts upon me, brings home to me.

3. Wound me not with thine eye, but with thy tongue:

Wound me not with thine eye = do not use the power of your eyes to harm me. See the notes to lines 12 and 14 below, which elaborate on the fatal power of the eyes in the contest of love.
but with thy tongue = use your tongue to wound me (by telling me that you have another lover).

4. Use power with power, and slay me not by art,

Use power with power - of uncertain meaning. Probably it means 'be directly forceful, overcome me by force (rather than stratagem)'. It could also imply 'use the weapons that I understand, fight me on my own terms'.
slay me not by art = do not kill me by using clever tricks and stratagems, but kill me directly (as in line 14).

5. Tell me thou lov'st elsewhere; but in my sight,

elsewhere = someone else, i.e. your heart is attached elsewhere, in someone else's bosom. The secondary meaning is 'not now, not in this place (when I am so unprepared to deal with the bad news)'.
in my sight = while I look on.

6. Dear heart, forbear to glance thine eye aside:

Dear heart = a term of endearment. As in Wyatt's famous poem:
Thanked be to Fortune, it hath been otherwise
Twenty times better; but once in special:
In thin array, after a pleasant guise,
When her loose gown from her shoulders did fall,
And she me caught in her arms long and small,
Therewith all sweetly did me kiss
And softly said, "Dear heart, how like you this?"
From the poems of Sir Thomas Wiat, edited by A.K.Foxwell, London 1913, Misc.poems 1528-36, No.11, 'They flee from me that sometime did me seek'. See elsewhere on this site for Wyatt's poems.  
It is possible that Shakespeare was echoing this verse with its endearment, remembering better times. The phrase is used elsewhere in the plays, but not frequently:
It is thyself, mine own self's better part,
Mine eye's clear eye, my dear heart's dearer heart, CE.III.2.61-2. Awake, dear heart, awake! thou hast slept well; Awake! Tem.I.2.305. Do, then, dear heart; for heaven shall hear our prayers; TIT.III.1.211. Farewell, dear heart, since I must needs be gone. TN.II.3. 97. Also in the Sonnets: Take heed, dear heart, of this large privilege; 95 Only in the Twelfth Night extract is it spoken with frivolity, in the others it is a genuine expression of tenderness.
forbear = refrain from, desist from (glancing aside).
to glance thine eye aside = to cast a sideways glance with your eye, i.e. give flirtatious glances to other men in the vicinity.

7. What need'st thou wound with cunning, when thy might

What need'st thou = why should you need to
wound with cunning = use cunning tricks to hurt me.
might = force, power. As in the phrase 'military might'.

8. Is more than my o'erpressed defence can bide?

The imagery of these two lines (7-8) is that of warfare, probably siege warfare. The cunning could consist of undermining the walls of the besieged town, or taking it by other Trojan horse type stratagems. The 'might' is the besieging army camped outside the walls.
o'erpressed defence = overstretched defences, defensive force which is barely able to withstand the onslaught.
bide = abide, endure, withstand.

9. Let me excuse thee: ah! my love well knows

Let me excuse thee - in a sudden volte face the poet now undertakes to do what he had refused to do in the opening two lines.
ah, my love well knows - a note of tenderness appears, appropriate to his change of heart. 9-12: The poet suggest that the true reason for his beloved's behaviour is consideration for him. She is sparing him the agony of turning her eyes in his direction, because she knows that in the past it has been devastating for him, as her eyes have not often, if at all, expressed approval of his love.

10. Her pretty looks have been mine enemies;

Her pretty looks = her lovely appearance, her bewitching glances.
have been mine enemies - because the beloved's looks always were potentially painful to the lover, as they could signify rejection, or disdain, and because, more specifically in this case, she has been obviously casting eyes in the direction of other men.

11. And therefore from my face she turns my foes,

from my face = away from my face.
my foes = mine enemies of the line above, i.e. her looks.

12. That they elsewhere might dart their injuries:

elsewhere = into someone else's heart; in another place (where I am not present and do not have to watch).
might dart their injuries = might send out their killing darts. The eyes alone of the beloved were capable of wounding the lover, but this was often allied with the idea of Cupid's darts, or arrows. Sidney has Cupid take up his residence in Stella's eyes, from which he sends out his darts to destroy onlookers' hearts. E.g. A&S.17:
........while Cupid weeping sate:
Till that his grandame, Nature, pitying it,
Of Stella's brows made him two better bows,
And in her eyes of arrows infinite.

13. Yet do not so; but since I am near slain,

Yet do not so = do not turn your eyes away from me.
near slain = nearly killed. Being wounded and slain by the beloved's eyes was relatively common in the love story as told by sonneteers. Compare for example Robert Tofte to Laura (published 1597):

And if, by chance, un'wares, thou sometimes kill :
Thou, with thy smile, the wound canst heal again ;
And give him life whom thou before hadst slain.

Laura, Pt.II.3.

14. Kill me outright with looks, and rid my pain.

Kill me outright with looks - This is fairly typical of the lover's approach to his stony hearted lover. (See the note above). Sidney calls Stella a thief, a murderer, a tyrant, a rebel runaway, a traitor, a witch and a devil in the final stanza of the Fifth Song, which sums up the accusations listed in the poem. This is the stanza which proves her to be a murderer:
Yet, English thieves do rob, but will not slay;
Thou, English murdering thief, wilt have hearts for thy prey;
The name of 'murderer' now on thy fair forhead sitteth;
And even while I do speak, my death wounds bleeding be,
Which, I protest, proceed from only cruel thee.
Who may, and will not, save, murder in truth committeth.
A&S.5th Song.49-54.
rid my pain = end my agonies, rid me of my suffering of unsatisfied longing for you.