To masturbate even after you've ejaculated.
Derek couldn't stop masturbating, thus commiting machine gun masturbation.
by B ryan October 05, 2006
When a man masturbates and when it seems he is finished he keeps going, thus making it hurt, this is machine gun masturbation.
Johnny was so horny he said he was going to use machine gun masturbation on his girlfriends face while she was sleeping.
by Lol'd on your mom October 05, 2006