Onomatopoeia for the beginning of "Sanford and Sons" theme song, often gets stuck in one's head and then said by that person to other friends so they can join in the fun.
Mitch: "hey corey and devon"
Devon and Corey: " whats up?"
Mitch: "Bwa-Bwa-Bwanup..."
Devon:"damnit, thats gonna be stuck in my headall day"
BWA = Bitch With Attitude. Referred to girls who think they can pass life with this kind of behavior. Also, those who are mean, lack of social skills and got serious attitude problems.