BCBB is an acronym for "Bowmp Chika Bowmp Bowmp". It is used on chat systems such as IRC (Internet Relay Chat). It's intended to mimic the stereotypical soundtrack for a pornographic movie, and is used to underscore real or imagined sexual innuendo.
<user1> user3 i don't want to hear about you bobbing on a navy ship
<user3> ha ha, user2, stfu.