What does "butter wouldn't melt in your mouth" mean — and why is it funny?

informal

Meaning

Describes someone who looks completely innocent and harmless — often with a hint that they are not.

Where it comes from

An old English phrase, in use since the 1500s. The idea is that such a person is so cool and demure that butter would not even soften in their mouth.

Why it is funny

The humor is the strange physical test it invents for innocence. To prove someone looks angelic, the phrase imagines checking whether butter melts in their mouth — and the unspoken wink is that the sweetest faces often hide the most mischief.

Used in a sentence

"She blamed her brother with a face like butter wouldn't melt in her mouth."