Cheeky Joke

Okay, so you know how an "ass" can mean both a donkey AND a butt? And "cheeky" means you're being a bit rude or funny? So, if you "crack a cheeky joke," you're telling a joke that's kinda naughty or funny, which makes the "ass" (the butt) laugh. You probably tell those kinds of jokes all the time, you silly goose!