What did shakespeare mean by And his unkindness may defeat my life But never taint my love.?

Answer 1

It is from this speech by Desdemona in Othello Act IV Scene 2

• O good Iago,
  What shall I do to win my lord again?
  Good friend, go to him; for, by this light of heaven,
  I know not how I lost him. Here I kneel:
  If e'er my will did trespass 'gainst his love,
  Either in discourse of thought or actual deed,
  Or that mine eyes, mine ears, or any sense,
  Delighted them in any other form;
  Or that I do not yet, and ever did.
  And ever will—though he do shake me off
  To beggarly divorcement—love him dearly,
  Comfort forswear me! Unkindness may do much;
  And his unkindness may defeat my life,
  But never taint my love.

Othello has by this point in the play been so twisted by Iago's machinations that he has called Desdemona a "whore" and a "strumpet" to her face. Desdemona goes to Iago of all people for help and guidance. Desdemona is as duped by Iago as Othello is. She says she cannot understand how she has lost Othello's love (oh, the irony that she should say so to Iago!). She protests, using the form "If I did any wrong then punish me" that she never did or thought anything against his love, never looked, listened or used any other sense to enjoy the company of any man but him, never stopped loving him dearly and never would, even if she was divorced and left bankrupt. If she has done any of this, she says, then "comfort forswear me", or in other words may she live in misery for the rest of her life. Then comes the sentence you ask about. "Unkindness may do much", that is, there are lots of nasty things that unkind people might do, and "his unkindness may defeat my life", meaning that out of his nastiness and cruelty, Othello might even kill her (a bit of foreshadowing there), but no matter how cruel and nasty he may be he will never taint her love, will never stop her from loving him completely and without reservation (even more foreshadowing).