ass
Explanation
The Joke
A person enthusiastically explains that the prefix "mega-" acts as an augmentative in Modern Greek, meaning that words like "megalodon" translate to "big tooth." Following this logic, they conclude that "megass" would mean "big butt." Their friend warns them not to say it too loudly -- but it is too late. The person shouts "YOU ASS-ASS!" and a massive explosion ("BOOM!") results, as though the word itself had destructive power.
The comic takes a real piece of linguistic trivia -- that "mega-" is a Greek augmentative prefix -- and pushes it to an absurd conclusion. The intermediate step (that "megass" means "big butt") is already silly enough, but the comic escalates further by having someone actually weaponize the insult, with the shout of "YOU ASS-ASS!" apparently being so powerful it causes a literal explosion.
The Humor
The humor comes from the collision of nerdy etymology with juvenile wordplay. The setup sounds like a genuine linguistics lesson, giving the punchline an air of false authority. There is also a visual gag in the escalation: the friend's worried warning creates tension, and the payoff is a comically over-the-top explosion triggered by what amounts to a double-butt insult. It plays on the idea that forbidden knowledge -- even something as silly as a Greek-derived butt joke -- can be dangerous if spoken aloud.