DIPPER, is pretty much a sexual manouvere. Basically to dipper something is like having sex with something but when you do the thrusting movement you dip downwards in a diagonal movement.
Also if you shout dipper and wave your hands above your head this means you are up for a dipper.
Calvin : Yo man DIPPPER (waves hands above head)
Sam and Adam : DIPPER (waving the hands above there heads and lunging)