Yak
An ugly female, or woman who is obese, or has unattractive features or an unpleasant odour. Has other meanings as well.
y
Yancey
A double meaning, where on the one hand it refers to a male who has a rather large penis. On the other hand, it refers to a heterosexual male who is acting in the perceived manner of a gay male, as sort of a warning.
Yancy
Newly coined slang word, referring to having a sexually transmitted disease.
Yaoi
Just like you would see in a directory listing, Yaoi would be the 'gay' listing. It refers to male on male pairings, in the Hentai or Ero anime section.
Yarbles
Slang reference to the male testes. Associated to a male's testicles, mainly due to the orb like shape of the testicles, hence the popular phrases.
You Know What
A double meaning. One in which it refers to having intercourse, where penetration occurs. That is the more popular of the two meanings. Another meaning is in reference to a woman's vagina.
Yuri
The Hentai category, this would refer to the female on female section of it, or in more Western Civilization terms, the Lesbian section.