short for "she's too young for you bro," the expression popularized by the TV show Jersey Shore, Jerry LaVigne Jr. and the facebook group "If she ______ she's too young for you bro" that collected hundreds of thousands of likes and tens of thousands of wallposts within only a few days.
"if you have to pretend your penis is a train or a plane to get it in her mouth styfyb"
"if she's looking up this definition, then styfyb!"
STYFYB is an acronym that was invented by a bored Facebook user to shorten the phrase " She's Too Young For You Bro!"
If she watches the movie the whole time, STYFYB
If she is amused by Nyan Cat, STYFYB
If she thinks 1337 is a year, STYFYN