Anonymous wrote:I always thought it meant super-smooth ... a real cool operator, maybe too cool for his own good. Here's Bill Safire's take ...
http://www.nytimes.com/1988/02/28/magazine/on-language-you-pays-yer-money.html?pagewanted=all&src=pm
According to Safire, not American or Southern at all, but rather British ...
The saying, first cited in John Palsgrave's 1530 book ... gained further currency in Jonathan Swift's writings .... In a 1781 comedy by Charles Macklin, the playwright used this expression in the sense it was used for centuries: ''She looks demure and good: and is less good and demure than she looks.'' The idea behind the phrase is that the person is so cold and proper that her mouth temperature would keep butter refrigerated and firm. ... The phrase has always been used sarcastically.
In recent years, however, a new sense has developed, threatening to push the old meaning out. ... The phrase now means ''eager to please, oleaginous in an attempt to curry favor.'' ... the phrase has lost its etymological moorings, perhaps having sucked some meaning out of buttery, ''oily, unctuous,'' or to butter up, ''flatter.''... So when you say, ''Butter wouldn't melt in his mouth,'' you now confuse listeners. You can mean ''He's uptight and frigid'' in the old sense, or you can mean ''He's all peaches and cream,'' which is the opposite.
Seems the traditional usage is that the person is uptight on the outside, but perhaps hotter underneath. Not necessarily bad, but just a big difference in attitude, like the school-marmish librarian who goes wild at night. But some people now use (misuse?) it to suggest warm and friendly on the outside, but perhaps in a calculated sense, because the person underneath is manipulative and likely bad.
Interesting stuff.