What does "a wolf in sheep's clothing" mean — and why is it funny?

informal

Meaning

A dangerous or dishonest person who hides behind a harmless, friendly appearance.

Where it comes from

A very old image, found in the Bible and in Aesop's fables, of a predator disguised among the flock it intends to eat.

Why it is funny

There is a dark comedy in the disguise itself. You picture a wolf earnestly wearing a fluffy sheep costume, trying to look casual among the real sheep — a menace let down by a slightly ridiculous outfit.

Used in a sentence

"He seemed so generous, but he turned out to be a wolf in sheep's clothing."