Why do we say "Butter wouldn't melt in his/her mouth"?

Meaning:

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Background:

Let’s start with the meaning of this phrase, as that in itself is unclear.

Some sources, including Merriam-Webster, indicate that the idiom refers to someone who seems innocent on the outside when in reality a much more caustic nature hides beneath the façade. This seems to be a more recent interpretation and some sources claim it’s especially popular in America's Southern states. The Collins Dictionary, too, cites several instances where newspapers have employed the idiom in the not too distant past. In modern times it’s often shortened to “butter wouldn’t melt,” as the saying is so common the speaker can assume their audience knows the rest of the phrase.

Complicating this particular definition is the fact that people sometimes use the saying to mean just the opposite, indicating that someone is so innocent that butter wouldn’t melt in their mouth. WorldWideWords.org cites a humorous novel by Canadian satirist Thomas Chandler Haliburton:

“He looks so good, all the women that see him say, ‘Ain’t he a dear man?’ He is meekness itself. Butter wouldn’t melt in his mouth. He has no pride in him.” —Nature and Human Nature (1855)

Most linguists feel this is the reverse of the phrase’s meaning, however, and this use is generally considered an incorrect interpretation of it.

Another very common interpretation, found on Phrases.org, is that the saying refers to someone who’s incredibly cool (either under pressure or just chill in general) or who has such a cold personality that they can’t generate enough warmth to melt the dairy product in question.

This last usage seems to be the original definition of the idiom, first appearing in print in 1530. In his textbook on the French language, Jehan Palsgrave, a priest and tutor in the Court of Henry VIII, wrote, “He maketh as thoughe butter wolde nat melte in his mouthe.” From the context it’s clear that the person he’s referring to exhibits a cool nature, not a deceptive one. Scholars believe that Palsgrave didn't coin the idiom, though, and that it was in common use in England by the 16th century.

Unfortunately, the saying’s true origin is lost to time, since no earlier written record of it exists.

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