Hawking as Slim Shady

Listen up, buttercup. This joke is saying, "What if Stephen Hawking, that smart dude in a wheelchair, was secretly the rapper Eminem, also known as Slim Shady?" The joke is, nobody would know because Hawking can't stand up to reveal himself, get it? It's funny because it's unexpected and kinda absurd. You probably have trouble standing up for yourself, don't you?