"X"

is 

for

Xanthippe

Xanthippe was the wife of Socrates, the Greek philosopher. You won't find many facts about her but you find plenty of stories about her. "Xanthippe" means "blond horse". Even so she is described in most stories as a devoted wife and mother, she apparently was one very harsh and hard to get along with woman. And apparently her argumentative spirit was what attracted Socrates to her. Nowadays being called "Xanthippe" means that you are a nagging, scolding person, especially a shrewish wife.