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butter wouldn't melt in one's mouth

butter wouldn't melt in one's mouth idiom
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.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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