The idiom "butter wouldn't melt in someone's mouth" means that someone appears to be innocent or pure, but in reality, they are not. It is used to describe someone who is behaving in a way that seems very sweet and innocent, but they may actually be up to something mischievous or deceitful.
For example, if a child is caught stealing cookies from the cookie jar, but they deny it with an innocent look on their face, you could say "butter wouldn't melt in their mouth" to describe how they appear to be innocent, but you know they are not.
The phrase comes from the fact that butter is a soft and smooth substance that melts easily in the mouth. So, when someone says "butter wouldn't melt in their mouth," they are saying that the person appears to be very soft and smooth, like butter, but they are not as innocent as they seem.
• Her calm demeanor and polite words made it seem as though butter wouldn’t melt in her mouth, but underneath she was seething with anger.
• My little nephew always looks like butter wouldn’t melt in his mouth, but he’s actually quite mischievous.
• The politician stood on the podium, speaking smoothly as if butter wouldn’t melt in his mouth, but everyone knew he was lying.
• She looked innocent enough, but butter wouldn't melt in her mouth.
• After stealing the cookies, she acted like butter wouldn’t melt in her mouth when her mother asked her if she knew what happened to them.
• The actress was so convincing in her role that she made it seem like butter wouldn’t melt in her mouth, even though she was playing a notorious criminal.