In Shakespeare's Macbeth, Lady Macbeth worries that she is too feminine, too nice, in possession of too many female characteristics to bring her assassination plan to conclusion.
In your question, I think you are referring to Lady Macbeth's speech in Act 1.5.39-56, after she is told that Duncan will be spending the night in her castle, well before she drugs the grooms, etc. I could be wrong, but if I am the above answer does an excellent job of analyzing that scene, so I'll comment on this one.
In this passage Lady Macbeth asks the spirits to:
...unsex me here,
And fill me from the crown to the top top-full
Of direst cruelty. Make thick my blood.
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall, you murd'ring ministers,...
Lady Macbeth wants to be unsexed, turned into a man so she can be aggressive and ruthless like one, and to be filled with cruelty. The sense of thickened blood is that thick blood will stop female traits like kindness and pity, and remorse, from reaching her mind. She wants any bit of human nature that may keep her from killing Duncan to be stopped, as well. She wants her mother's milk to be turned into poison.
Lady Macbeth is afraid she won't be able to go through with it. Of course, as far as planning the assassination and convincing Macbeth to do it, she has more than enough ambition, evil, and ruthlessness to go through with it, and she does. But when she has an opportunity to kill Duncan herself, she is, at least, too sentimental to do it. The sleeping Duncan reminds her of her father, and she is not able to use the daggers on him herself.
Lady Macbeth cannot distance herself from the act of murdering Duncan because the King of Scotland resembles her father as he sleeps:
Alack, I am afraid they have awaked/And 'tis not done! Th'attempt and not the deed /Confounds us. Hark! I laid their daggers ready;/He could not miss 'em. Had he not resembled/My father as he slept, I had done 't. (II,ii,9-13)
This passage is very telling, indeed, because it indicates the vulnerability to guilt that Lady Macbeth possesses. First, she worries that the deed of slaying Duncan will "confound," or ruin her husband and her. Then, she indicates human sympathies as she remarks that the sleeping Duncan resembles her father, whom she obviously has loved. In addition, her later words with her husband ring hollow as she accuses him of wearing a "heart so white" (II,ii,64), and she tells him that "A little water clears us of this deed" (II,ii,66). This show of bravado by Lady Macbeth is in direct contradiction to her earlier feeling that they would be ruined by the deed.
In Act II, the lines of Lady Macbeth foreshadow her forthcoming unraveling as she becomes insane from her guilt in Act V as they also enhance the theme of guilt.