butter wouldn't melt (in his/her mouth), looks as if

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.
See also: butter, melt

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!
See also: butter, melt

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.
See also: butter, if, look, melt