Boomerang of Fear

Listen up, nimrod! This joke is dark. The dude threw a boomerang, right? Now he's scared because it could come back and hit him at any moment. It's funny because it implies he's been waiting for years for the boomerang to return and possibly kill or hurt him. The joke is about the fear and uncertainty of waiting for something bad to happen. You probably still throw rocks at your friends, don't you?