Cunt may or may not be one of the worst words in English at the moment, but it's subject to change over time and on the situation. To really know how a swear word is used, you also need to know the context as to why it is "bad". I remember when I was growing up faggot (homosexual person, but literally a bundle of sticks or a cigarette) or queer (homosexual, literally strange or different) was probably among the worst words you could use, but now because homosexuality is more accepted, they aren't that bad. Teenage boys use the word "fag" every five minutes now.
Cunt is probably the among worst swear words in English at the moment, because many consider it sexist against women, and sexism is highly disapproved of most places in North America. It could be subject to change, especially considering that feminists are "reclaiming" the word as being a valid word for women to use when talking about their own vagina. Not unlike how the homosexual community reclaimed the word queer.
>>32 Cunt also means female genitalia. The meaning of nasty comes from the idea that female genitalia or their owners are frequently unpleasant.