Bostin is a Brummie slang term, which is to say it's peculiar to residents of Birmingham, England, and it means "excellent" or "awesome" or "Wicked" or "cool beans." I picked it up here on LiveJournal, from a Harry Potter role-playing community, of all places.
The community in question was called Semper Arcanum. It's no longer a going concern; it petered out in the summer of 2005, apparently by common consent of the players. One of the characters was Michael Corner, a Ravenclaw in Harry's year who's a very minor character in the books but was fairly important in this particular game. He and Ginny Weasley engaged in an on again-off again flirtation over the course of the year the RPG took place; I remember one post in which he and she traded cheesy vintage valentines back and forth in the comments.
Anyway, one of the conceits adopted by the person who played Michael was that he was from Birmingham, and therefore his posts and comments were liberally sprinkled in Brummie slang, and for whatever reason I took a shine to bostin and incorporated it into my own vocabulary. I keep expecting someone to ask about it, because it's not a word most people know, but since the meaning is always clear from context, I guess most people don't care why I said it. Maybe people just expect me to say weird, incomprehensible things from time to time.