This phrase means to be prim and proper, or demure and insincere, with a cool demeanor, the cool demeanor being the most important part. It is an old saying, and was included as a proverb in John Heywood’s collection of 1546.
The saying is not easy to understand, since in these modern days of refrigerators, you could put butter in your mouth straight from the freezer and it would quickly melt. The saying can only be applied to people who are so cool and emotionless that they don’t even have enough warmth to melt butter.
A typical use appears in William Makepeace Thackeray’s Pendennis: “When a visitor comes in, she smiles and languishes, you’d think that butter wouldn’t melt in her mouth: and the minute he is gone, very likely, she flares up like a little demon, and says things fit to send you wild.”
The phrase is usually used in a derogatory sense but sometimes denoted a meek sweetness of temper rather than emotional coldness. An example is this description of Mr Pecksniff in Charles Dickens’ Martin Chuzzlewit: “It would be no description of Mr Pecksniff’s gentleness of manner to adopt the common parlance, and say that he looked at this moment as if butter wouldn’t melt in his mouth. He rather looked as if any quantity of butter might have been made out of him, by churning the milk of human kindness, as it spouted upwards from his heart.”
Since I’m quite happy to have butter melting in my mouth or over my baked potato, let me list a few other food-related phrases that simply don’t have enough history to warrant a whole page to themselves.
Cry over spilt milk — to be upset over a past event that can’t be changed
Too many cooks spoil the broth — too many chiefs, not enough Indians (16th C)
Pig’s breakfast or dog’s dinner — unappetizing, distasteful, a mess, a muddle
Under your belt — food consumed or something satisfactorily achieved
Have a finger in every pie — be involved in a lot of different activities. Shakespeare used it in Henry VIII:
“The devil speed him!
No man’s pie is freed
From this ambitious finger!”
To die for — how I feel about pie. Pecan. Lemon meringue. Chocolate. Pumpkin. Apple.