Idioms and Phrases with butter wouldn't melt in one's mouth
butter wouldn't melt in one's mouth
Be overly coy or demure; be insincere. For example, She looked quite innocent, as though butter wouldn't melt in her mouth, but we knew better. Already a proverb in John Heywood's collection of 1546, this metaphoric expression alleges that one is literally so cool that butter inside the mouth would not melt.