What does "a leopard can't change its spots" mean — and why is it funny?

informal

Meaning

People cannot change their basic nature or character.

Where it comes from

A phrase from the Bible, in the Book of Jeremiah, using the leopard's permanent markings as an image of a fixed nature.

Why it is funny

The mild humor is the vanity it hints at. The phrase pictures a leopard who might genuinely like to swap its spots for something new, and gently informs it — and us — that this is simply not on offer.

Used in a sentence

"He promised to be on time, but a leopard can't change its spots."