You know how Stephen is smart, which class did he skip?
Leg day.
Alright listen up, buttercup. The joke is that Stephen Hawking was super smart but also had a physical disability that kept him from, you know, doin' leg day at the gym. So, they're sayin' he skipped "leg day" which is also like sayin' he skipped a class. Get it? I didn't think you would.