Butter wouldn’t melt in his mouth

This phrase “so sweet that butter wouldn’t melt in his mouth” first appears in print in Proverbs by John Heywood, printed in 1546, although a similar proverb appeared in a French volume published some 15 years earlier. Neither source, of course, did more than report a saying already widely popular among the common people. The sense in which it is usually used implies that, even though the person so described looks innocent, you had better beware because looks frequently are deceiving and the person may, in the words of one writer, be “suspiciously amiable.”