It's a shortened version of Babe or Bitch, depending on who you're talking to and how you mean it.
Girl 1: Oh hey B, what's going?
Bitch: Not a lot Babe, wbu?
Guy 1: Wuddup B?
Babe 1: Not a lot boy.
Guy 2: Wow B you're gonna get it tonight...
Guy 3: Ya Bitch is going down!