The acronym that stands for Boob to Frame ratio. Its a mathematical ratio designed to determine how big a woman's breasts are compared to what her frame should be able to handle. It is used to discern how attractive a girl is when cup size doesn't tell the whole story.
Matt: Dude my girlfriend has bangin double d's!!
Ryan: Yeah... but she has shoulders like a linebacker, sorry bro but her BTF is waaaay too small for me.
On the Big Island of Hawai'i, down in the lower Puna District,
BTF means Below the Flow. Someone or something that is located below the flow of Tutu Pele's path.
A person who exemplifies a body crafted by the god's hands to service mankind's ultimate fantasies of beauty and lust. This person is has absolutely no trouble finding sex in any scenario. Everyone wishes to look like this person, and everyone wishes to fuck this person. The person has all exclusive rights to chose their fuck buddy for the evening, because when chosen, you must not turn down the opportunity. If you do, you will be thought of as queer or just flat out retarded. This person is so well versed in intercourse that even a nerdy virgin would turn into a sex master (or at least an intermediate) just by (somehow) having intercourse with this person. You will know this person when you see them; they are the orgasm in your eye.
WARNING: while this person may be BTF, they may not be DTF. (some people are assholes like that)