What does "mad as a hatter" mean — and why is it funny?

informal

Meaning

Completely crazy or wildly eccentric.

Where it comes from

Often traced to the 1800s hat-making trade, where mercury used to treat felt poisoned hatters and harmed their minds; Lewis Carroll's Mad Hatter later fixed the phrase in everyone's memory.

Why it is funny

The comedy now is the innocent, old-fashioned job at the centre of it. 'Hatter' sounds gentle and quaint, so yoking it to raving madness is an odd, funny mismatch — and Carroll's chaotic tea party keeps the picture alive.

Used in a sentence

"He wants to cross the ocean in a bathtub — mad as a hatter."