Why, butter would not melt in his mouth!

Understand the thought

Meaning in plain English

An apparently innocent or composed manner can invite scepticism about what someone has done. The butter image is a pointed comment on appearances, not proof that a quiet person is deceptive.

What the words are saying

The exclamation pictures someone as so outwardly mild that butter would not melt in their mouth. In a figurative reading, that manner seems at odds with an action the speaker suspects or knows about. The saying can express irony when a person presents an innocent face after causing mischief. Its usefulness depends on evidence beyond the expression itself. A calm voice, reserve or unfamiliar manner cannot establish dishonesty, and repeating the phrase may unfairly turn an impression into an accusation. It is best understood as a comment on a perceived gap between conduct and presentation, rather than a reliable method for judging character.

Everyday example

A friend admits rearranging the game pieces as a joke, then puts on an exaggerated innocent expression that prompts the comment.

When to use it

Use it for a known contrast between an innocent presentation and an acknowledged action.

Agent-assisted editorial interpretation. This explains the words; it does not by itself verify their author or earliest origin.