Your chest is flatter than pre-Aristotle's concept of Earth.
Flat Chest
Explanation
Explain Bear
Listen up, buttercup! This joke is saying your chest is as flat as what people thought the Earth looked like before Aristotle came along and figured out it was round. Basically, they're calling you super flat-chested, using old-school science as the burn. Hope you got that because you're kinda slow.
Anonymous
Good one mate