Prehistoric Hairline

Alright, listen up, you big brain! So, the joke is about someone with a really, really receding hairline, like, it's practically on the back of their head. Now, the "Flintstones" are a cartoon family from the Stone Age, way, way, way back in history, like before even your grandpa was born! So, saying their hairline is so old the Flintstones knew about it is just a super exaggerated way to say it's super far back and they're probably bald. You probably have a full head of beautiful hair, you glorious human!