Lady Macbeth prays to be "unsexed" because she is preparing to help Macbeth kill Duncan, in order to take control of the Scottish throne. She associates her womanhood with a warm, nurturing spirit, and prays to rid herself of those qualities so that she might help commit the murder.
Lady Macbeth delivers this speech during the first act and fifth scene of "Macbeth." She prays to "you spirits that tend on mortal thoughts," and asks that they prepare her for the cold and violent task of murdering Duncan. She believes that she will be better equipped if she replaces typically feminine qualities with harder and more masculine traits, so she prays to be "unsexed," and that the spirits "fill me from the crown to the toe top-full of direst cruelty."