butter wouldn't melt in someone's mouth

alternative butter will not melt in someone's mouth · alternative butter won't melt in someone's mouth · alternative butter would not melt in someone's mouth

1. idiomaticThe 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, Fergus Hume, “‘Enter Mrs Pansey as Chorus’”, in Bishop Pendle: Or, The Bishop’s Secret, Chicago, Ill.; New York, N.Y.: Rand, McNally & Company, →OCLC, page 7

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   Col. Why, they say she's one of the chief toasts in town. Lady S. Ay, when all the rest are out of it. Miss. Well; I wouldn't be as sick as she's proud for all the world. Lady A. She looks as if butter wouldn't melt in her mouth but, I warrant, cheese won't choke her. . . . Col. I can't pardon her for her rudeness to me.

   1738, Jonathan Swift, “Polite Conversation”, in The Works of Jonathan Swift, D. D., volume 6, published 1859, page 45

   When a visitor comes in, she smiles and languishes, you'd think that butter wouldn't melt in her mouth: and the minute he is gone, very likely, she flares up like a little demon, and says things fit to send you wild.

   1850, William Makepeace Thackeray, chapter 61, in The History of Pendennis

   "Yes, he is a ruffian and a brute, and I don't see what Mr. Cameron sees about him to like, I am sure." "Probably the boy makes him think he is a model of excellence. Such boys are apt to be deceitful." "He's deceitful enough. You'd think butter wouldn't melt in his mouth."

   1875, Horatio Alger, chapter 29, in Herbert Carter's Legacy

   "Evil, wise and cruel," reflected Rob, as he restored the spectacles to his pocket. "How easily such a man could impose upon people. To look at him one would think that butter wouldn't melt in his mouth!"

   1901, L. Frank Baum, chapter 18, in The Master Key

   "There's old Stephen Grant coming in," exclaimed Peg viciously, shaking her floury fist at him, "and looking as if butter wouldn't melt in his mouth. He may be an elder, but he's a scoundrel just the same."

   1913, Lucy Maud Montgomery, chapter 21, in The Golden Road

2. idiomaticThe identified person is prim and proper, standoffish, cool, or dispassionate.

   Sir Robert is the type of whom it is said that butter wouldn’t melt in his mouth; Mr. Nivola invests him with such a potent blend of froideur and hauteur that you imagine that butter wouldn’t melt anywhere within a 10-foot radius of him.

   2013 October 17, Charles Isherwood, “‘The Winslow Boy’ Is Revived at American Airlines Theater”, in New York Times, retrieved 12 Jul 2015

   2 more examples

   All unmarried women are necessarily in the market; but if they behave themselves properly they make no signs. Now there was Griselda Grantly; of course she intended to get herself a husband, and a very grand one she has got: but she always looked as though butter would not melt in her mouth.

   1861, Anthony Trollope, chapter 38, in Framley Parsonage

   It was a very long way from Mozart's Pamina, Handel's Alcina, Donizetti's Adina or Bizet's butter-wouldn't-melt-in-her-mouth Micaela. Indeed, Lisa Milne had put away her shining soprano for the night. . . . The only way to reach the songs of Jacques Brel is from the heart; the only way to deliver them, from the gut.

   2004 October 21, Edward Seckerson, “Lisa Milne Sings, Jacques Brel, Wigmore Hall, London”, in Independent, UK, retrieved 12 Jul 2015