look as if butter wouldn't melt in your mouth

look as if butter wouldn't melt in (one's) mouth

To appear innocent or reserved in certain company, when in fact one may have the opposite demeanor. Sure, he looks as if butter wouldn'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.
See also: butter, if, look, melt, mouth
Farlex Dictionary of Idioms.

look as if butter wouldn't melt in your mouth

appear deceptively gentle or innocent. informal
See also: butter, if, look, melt, mouth
Farlex Partner Idioms Dictionary
See also:
  • look as if butter wouldn't melt in (one's) mouth
  • look as if butter wouldn't melt in mouth
  • butter wouldn't melt
  • butter wouldn't melt (in one's mouth)
  • butter wouldn't melt in one's mouth
  • butter wouldn't melt in someone's mouth
  • butter won't melt (in one's mouth)
  • devil is not so black as he is painted
  • more to the point
  • eye up