butter won't melt (in one's mouth)(redirected from butter will not melt in mouth)
butter won't melt (in one's mouth)
Said of one who appears innocent, modest, or reserved in certain company, when in fact one may have the opposite demeanor. Sure, he looks as if butter won't melt in his mouth when he's around people he doesn't know, but stay a while and you'll see what he's really like. Samantha always affects this demure, butter-won't-melt expression on her face whenever she wants something from you.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.