butter wouldn't melt in (someone's) mouth

Translations
масло не растает во рту (у кого-то)
She looked as if butter wouldn't melt in her mouth.
Она выглядела так, будто масло не растает у неё во рту.
Definitions
This phrase is used to describe someone who appears innocent, demure, or sweet, but may actually be deceitful or manipulative. It suggests that the person is putting on a facade of purity or gentleness while hiding their true nature.