Definition & Meaning of "butter would not melt in one's mouth"in English

What is the origin of the idiom "butter would not melt in one's mouth" and when to use it?

The idiom "butter would not melt in one's mouth" originates from the idea that if someone's demeanor or behavior is as innocent and pure as butter, it would not melt even in their mouth. The phrase is primarily used to convey the idea of someone appearing or pretending to be extremely innocent, demure, or well-behaved, despite potentially possessing a deceptive or mischievous nature. In other words, it is used to describe individuals who feign innocence or act in a way that hides their true intentions or mischievous nature.