beating a chimp

While spanking the monkey defines masturbating, beating a chimp is getting punched in the balls multiple times, causing you to make chimp like noises

Beating a chimp: to punch the male genitalia at a rapid pace

Pete: Dave broke up with his girlfriend
Bob: how'd that go?
Pete: well, she beat a chimp
Bob: what's that mean?
Pete: well all i can say was his balls hurt afterwards