droppin a beat
To drop ones pants and masturbate profusely as in making music. In certain atmospheres it is popular to masturbate to the music of the pornography therefore dropping a beat. A.K.A Jacking Off, Masturbating, Choking the Chicken, Spanking the Monkey
Bob: Yo dude I heard this weird smacking noise in your room.
Jim: Yeah dude I was droppin a beat.
Bob: Gross you better not have gotten some milk juice on my                 keyboard