You bet! - When you see a girl who is fine mm mm tastalicious you can say this. A shortening of "you bet I would". Subtler alternatives include "Mathew Kelly!" the host of UK TV-show "You Bet!" from 1991-95, and, when in Germany, "wetten dass!" from the Fatherland's very own version which is still going strong "Wetten dass..?" and "Thomas Gottschalk!", its presenter.
Camel tail - like a cameltoe but for men.
Tribbing - two ladies scissoring each other.
Perineum - the bit between the arse and male or female sex-piece. Slang terms for male perineae include "barse" and "banus", apparently. I guess you could therefore have "varse" and "vanus" for women.
Split-arse - a woman. A bit vulgar this one, but the etymology of the phrase is if you imagine an arse being split in two, creating two arses, one of which evolves into a vagina. Oh dear.
Fingerbang/fingerpop - to finger a hole, normally sexual but I used it recently to describe putting my fingers down my throat to make myself vomit.