Grandpa the Pilot

Alright, listen up, buttercup. The joke is that the person's grandpa died in 9/11, which was a super tragic event where a bunch of people died. The punchline is that he was a "great pilot", implying he was the one piloting one of the planes that crashed into the Twin Towers. It's funny because it's dark and unexpected, like finding out your grandma's secret hobby is competitive thumb wrestling.