An aggressive expression yelled when something exciting goes your way usually at someone else's expense.
The phrase evolved from guy who got a blowjob from a girl he didn't know on the bank of a bay. After she was done, she dropped her ring, bent down to pick it up and the guy kicked her in the ass as she flew face first right into the muddy water. He yelled, "BAM BITCH GET IN THE BAY!" Then he left.
You bet your whole bankroll in vegas on one black jack hand. You are showing 18 and the dealer shows a king. He flips a 4 then busts with a 9. In this case, it is very appropriate to yell, "BAM BITCH!"