Perhaps one of the best euphemisms for masturbation
. Used rather effectively in American Beauty
Carolyn: What are you doing?
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!