( informal ) used to say that sb seems to be innocent, kind, etc. when they are not really
BUTTER WOULDN'T MELT (IN SB'S MOUTH)
Meaning of BUTTER WOULDN'T MELT (IN SB'S MOUTH) in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005