What does "nutty as a fruitcake" mean — and why is it funny?

informal

Meaning

Very eccentric, foolish, or crazy.

Where it comes from

American slang from the early 1900s, playing on 'nutty' meaning both literally full of nuts and slang for crazy.

Why it is funny

The humor is the pun made out loud. A fruitcake really is packed with nuts, so the phrase lets a dense, lumpy dessert stand in for a person's state of mind — and the comparison is as rich and chaotic as the cake.

Used in a sentence

"My uncle keeps a pet goose in the kitchen — nutty as a fruitcake."