Last Words

Alright, listen up, buttercup. The joke is funny because the person is recalling their mother's last words, which you'd expect to be profound or sentimental. But instead, her last words are a question about why her child is holding a sledgehammer, implying they're about to do something violent to her. It's dark humor because it makes light of a very grim situation. You probably think a sledgehammer is something from a hardware store instead of a murder weapon, you silly goose!