The Rice University Neologisms Database

Sexual relations The term "the nasty" is used primarily because of its informal, casual tone and also to lessen the seriousness of such an act. However, because the term is slang and refers to sexual relations in a much more casual and trivial sense it can seem disrespectful and is only to be used with very close friends. Because the term refers to doing something gross (nasty) it degrades the act of sex, which some people may prefer and some may be opposed to. It is probably derived from the word meaning "dirty" and "foul" because of the fact that is usually refers to one night stands or to very casual sex, which would make such an act something to be ashamed of and something to cover up (such as something literally dirty). I have only heard the term "the nasty" used by young adults (high school and college students) and it is often said in the stereotypical "gangster" voice and with a tough demeanor. I believe the person who coined this term chose to use this as opposed to something already established such as "sex" because it has a hint of comedy and because it makes it less shameful.