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. Samantha always affects this demure, butter-wouldn't-melt expression on her face whenever she wants something from you. I know you think that Erin's just pulling a "butter wouldn't melt in my mouth" act, but now that I've spent more time with her, I think she genuinely is shy.

butter wouldn't melt in someone's mouth

If you say that butter wouldn't melt in someone's mouth, you mean that although they look completely innocent, they are capable of doing bad things. He may look as though butter wouldn't melt in his mouth, but I wouldn't trust him. To look at her you'd think that butter wouldn't melt in her mouth. You wouldn't dream she'd even heard language like that. Note: You can also say that butter wouldn't melt. Mums and dads who see their sweet little Sally Ann tripping off for a night's studying at the chemistry society, looking like butter wouldn't melt, would be advised to look closer.