Bring My Beer. Boys got a tendency to be kind of self centered and just think BYOB (bring your own beer). However, this term, BMB, are used by handsomegentlemen who also think about the ladies and bring their beer as well.
Bmb stands for "be my boy" for straight forward girls that like to be the one to ask first and the positive answer ( from a boy in this case is "yes I'll byb" be your boy
Or if it was the opposite way around it would be bmg which stands for " be my girl ".