A fork of Rural Dictionary
ISB is an acronym for Incredibly Sexy Butt. It has been used on numerous occasions toward people to post pictures of their sexy butts. Non bare of course. People have been known to wear pajama pants, jeans, etc. Has been used on forums as a joke to get members involved by posting pictures of sexy butts. It's good fun.
Oh my god, you have a great ISB", "Post your ISB, everyone is doing it.