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

look as if butter wouldn't melt in (one's) mouth

To appear 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, if, look, melt, mouth
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

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
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.

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
Farlex Partner Idioms Dictionary © Farlex 2017

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
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
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