butter wouldn't melt in someone's mouth
📖 generalMeaning
Said of someone who appears innocent, demure, and harmless but is, in fact, often insincere, sly, or mischievous.
Literal Meaning
The person's mouth is so cold that even butter would not melt when placed inside it.
Example
“She looked so sweet and innocent, like butter wouldn't melt in her mouth, but she was the one who started all the rumors.”