butter wouldn't melt in one's mouth

butter wouldn't melt (in (one's) mouth)

Said of one who appears innocent or reserved in certain company, when in fact one may have the opposite demeanor. Sure, he looks as if butter wouldn't melt in his mouth when he's around people he doesn't know, but stay a while and you'll see what he's really like.
See also: butter, melt

butter wouldn't melt in one's mouth

Be overly coy or demure; be insincere. For example, She looked quite innocent, as though butter wouldn't melt in her mouth, but we knew better . Already a proverb in John Heywood's collection of 1546, this metaphoric expression alleges that one is literally so cool that butter inside the mouth would not melt.
See also: butter, melt, mouth