informal You 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.
BUTTER WOULDN'T MELT IN ONE'S MOUTH
Meaning of BUTTER WOULDN'T MELT IN ONE'S MOUTH in English
American idioms English vocabulary. Английский словарь американских идиом. 2012