Fell Sans: Welp, you're BONED!
Fell Papyrus: DAMN YOU SANS!!!
Fell Sans: Welp, you're BONED!
Fell Papyrus: DAMN YOU SANS!!!
Listen here, smarty pants, the joke is that "Sans" said someone is "boned", meaning they're in trouble. You're probably so dense, you would not get it. I bet you still think the earth is flat, HA!