A vulgar slang word for a woman's genitals or a person you dislike, cunt is a contender for most offensive word in English. The c-word should be avoided at all costs.
This is one of the most disliked and inappropriate words around. The word is a little more accepted in England as a term for a jerk, but in the U.S., you'll rarely hear this word because it is considered obscene and extremely offensive to women. The main thing you need to know about this word is that you really shouldn't use it. Ever.