It's so sad that Stephen Hawking has a whole category on here about him and he can't stand up for himself.
Categories
Explanation
Explain Bear
Listen up, buttercup. The joke is that Stephen Hawking, bless his soul, couldn't stand up because he had a disease called ALS. So, when the joke says he "can't stand up for himself," it's playing on the double meaning. You probably thought he was just lazy!