What does "cost an arm and a leg" mean — and why is it funny?

informal

Meaning

To be extremely expensive.

Where it comes from

Emerged in American English after World War II, possibly echoing the real cost of war, and built on older expressions about giving body parts for something valuable.

Why it is funny

It is funny through grim exaggeration. No shop takes limbs as payment, so the phrase prices an object in something priceless and irreplaceable. The dark, deadpan overstatement — as if you really would dismember yourself for a new sofa — is the joke.

Used in a sentence

"That concert ticket cost an arm and a leg."