butter wouldn't melt in someone's mouth

phrase2

1. •
   The identified person appears to be benign, mild-mannered, or calm (but with an integral suggestion that, to the contrary, they really are untrustworthy, disagreeable, or mean-spirited).

   • Yes, he is the bishop's chaplain; a Jesuit in disguise I call him, with his moping and mowing and sneaky ways. Butter wouldn't melt in his mouth; […] — 1900
2. •
   The identified person is prim and proper, standoffish, cool, or dispassionate.