butter wouldn't melt in someone's mouth

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

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butter wouldn't melt in someone's mouth

1. (idiomatic) The identified person appears to be benign, mild-mannered, or calm (but with an integral suggestion that, to the contrary, he or she really is untrustworthy, disagreeable, or mean-spirited).
   • 1738, Jonathan Swift, "Polite Conversation" in The Works of Jonathan Swift, D. D., vol 6 (1859 edition), p. 45 (Google preview):

     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.

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

     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.

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

     "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."

   • 1900, Fergus Hume, chapter 1, in The Bishop's Secret:

     "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."

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

     "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!"

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

     "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."

   • 1918, Edgar Wallace, chapter 2, in The Man Who Knew:

     "I've often laughed, seeing you walk past me as though butter wouldn't melt in your mouth and everybody saying what a nice young man Mr. So-and-so is, and I have thought, if they only knew that this sleek lad—"

     "Shut up!" said the other savagely.

   • 2015 Feb. 5, Julian Robinson, "'Angelic' looking teen who terrorised his village," Daily Mail (UK) (retrieved 13 July 2015):

     One villager said: "This lad looks like butter wouldn't melt in his mouth with his blonde hair and good looks. But he is no cherub in real-life. He is the devil child for many people around here."

2. (idiomatic) The identified person is prim and proper, standoffish, cool, or dispassionate.
   • 1861, Anthony Trollope, chapter 38, in Framley Parsonage:

     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.

   • 2004 Oct. 21, Edward Seckerson, "Lisa Milne Sings, Jacques Brel, Wigmore Hall, London," Independent (UK) (retrieved 12 July 2015):

     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.

   • 2013 Oct. 17, Charles Isherwood, "‘The Winslow Boy’ Is Revived at American Airlines Theater," New York Times (retrieved 12 July 2015):

     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.

• butter wouldn't melt (in someone's mouth) at The Free Dictionary
• butter wouldn't melt in someone's mouth at MacMillan Dictionary
• discussion of the variations in meaning of "butter wouldn't melt in his mouth" at World Wide Words
• discussion of the origin of "butter would not melt in her mouth" at MIStupid.com
• discussion of this phrase in "On Language" column in New York Times (28 Feb 1988) by William Safire