butter wouldn't melt
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.
butter wouldn't melt (in someone's mouth)
Prov. Someone is acting as if innocent. By the time her parents came home, Emily had cleaned up all evidence of having broken the valuable figurine, and she looked as though butter wouldn't melt in her mouth. Jane: How can you suspect George of playing that practical joke on you? He looks so innocent. Jill: Yes, butter wouldn't melt, I'm sure.
ˌbutter wouldn’t ˈmelt (in somebody’s ˈmouth)(spoken) a person looks very innocent, but probably is not: She looks as if butter wouldn’t melt in her mouth, but don’t be fooled by first impressions!
butter wouldn't melt (in his/her mouth), looks as if
Suspiciously coy or demure, too good to be true. This expression dates from the early sixteenth century and was already a proverb in John Heywood’s 1546 collection. It recurs again and again (Swift, Dickens, Thackeray, Shaw) and has been a cliché for at least a century.