I'm pretty sure I've worked it out, but I've never seen it used before aside from the sound Nightcrawler makes. It is the phrase on Sam Jackson's wallet in Pulp Fiction, yes?
Ah, I misremembered. Just watched the scene again. The wallet just has "Bad motherfucker" on it. But yeah, that's what I had pieced together. BAMF will always be Nightcrawler to me, though.