Ad hominem, in Latin, means "to the person": it is an argumentative strategy used by someone who avoids the actual topic under discussion and, instead, attacks the person who makes the argument. Lady Macbeth employs this tactic immediately after Duncan's murder. Macbeth is concerned about the potential negative consequences of having killed the king (who is also his guest, cousin, and good friend), and Lady Macbeth, rather than deal with his concerns, attacks him as weak and cowardly. He worries that he will never be able to sleep peacefully again, having murdered someone while they slept, and she responds,
Why, worthy thane,
You do unbend your noble strength to think
So brainsickly of things. (2.2.58-60)
She accuses him of coming unraveled, losing his cool, being weak in the face of this stress. She suggests that he has the strength to keep it together but that he is allowing himself to dwell and be morbid. Her arguments increase when he refuses to return the daggers to the room. She says that he is "Infirm of purpose" and that it is "the eye of childhood / That fears a painted devil" (2.2.68, 2.2.70-71). She claims, again, that he is weak and acting like a child. She must return the daggers herself, getting blood on her own hands. When she returns, she tells him,
My hands are of your color, but I shame
To wear a heart so white. (2.2.82-83)
Macbeth fears that this terrible guilt, symbolized by the blood on his hands, will follow him forever, that he will never be able to do anything to remove its figurative stain. Lady Macbeth tells him that her hands are bloodied now too, but she would be ashamed to have a heart which is as cowardly as Macbeth's. Time and time again in this scene, Macbeth presents his very real fears about the consequences of murdering Duncan, and Lady Macbeth suggests that this makes him both a child and a coward.