Why, butter would not melt in his mouth!

The person appears so mild and innocent that he seems incapable of doing anything wrong. The image of unmelted butter exaggerates that impression of harmlessness.

The remark is typically skeptical rather than admiring: his innocent manner may be a pose that conceals mischief, dishonesty, or a willingness to cause trouble.

Use the proverb when someone presents himself as blameless but you suspect his behavior tells a different story. It can also follow an account of wrongdoing to point out the contrast between a person's gentle appearance and his actions.