- butter\ wouldn't\ melt\ in\ one's\ mouth
- informalYou act very polite and friendly but do not really care, you are very nice to people but are not sincere.
The new secretary was rude to the other workers, but when she talked to the boss, butter wouldn't melt in her mouth.
Словарь американских идиом. — СПб., Изд-во "Лань". Оригинал: A.Маккей, М.Т.Боткер, Дж.И.Гейтс. 1997.