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.
Words nearby butter wouldn't melt in one's mouth
butter someone up,
butter spreader,
butter tart,
butter tree,
butter up,
butter wouldn't melt in one's mouth,
butter-and-egg man,
butter-and-eggs,
butterball,
butterbread,
butterbur