Perhaps one of the best euphemisms for
masturbation. Used rather effectively in
American Beauty:
Carolyn: What are you doing?
Lester: Nothing.
Carolyn: You were masturbating!
Lester: I was not.
Carolyn: Yes you were!
Lester: Oh, all right! So shoot me, I was whacking off! That's right, I was choking the bishop, chafing the carrot, you know, saying "hi" to my monster!