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.