Why do we say "Keep it under your hat"?

Background:

The phrase “keep it under your hat” is an admonition for you to keep something secret. An example of this might be something like, “My roses are award-winners because I use this special fertilizer blend – but keep it under your hat!”

It’s often stated that this saying originated in Medieval England, in the days when wars were fought with swords and longbows rather than guns and bombs. Some speculate the phrase arose from a custom employed by archers, who kept their bowstrings under their hats to keep them both handy and dry. While this was true, this practice almost certainly did not give rise to the idiom, as it doesn’t appear in print until much later.

Along these same lines, some attribute the phrase to U.S. President Abraham Lincoln’s habit of storing notes in the liner of his stovepipe hat (and Lincoln did, in fact, refer to his hat as his office from time to time). But again, while this may have been true, most linguists don’t believe the practice to have spawned the direction to keep something under one’s hat.

Instead, most feel the saying arose in mid-19th century England - a time and place when people commonly wore hats. To keep something under one’s hat simply meant, during the era, to keep information in one’s head. Phrases.org.uk cites the earliest use of this phrase in print in William Makepeace Thackery’s 1848 novel, The History of Pendennis:

Thus, oh friendly readers, we see how every man in the world has his own private griefs and business… You and your wife have pressed the same pillow for forty years and fancy yourselves united. Psha, does she cry out when you have the gout, or do you lie awake when she has the toothache? … Ah, sir – a distinct universe walks about under your hat and under mine.

It seems that when the saying “crossed the pond” to the United States “keep it under your hat” evolved into the idea of maintaining a secret. World Wide Words cites an 1892 article in the Medina, Ohio periodical, Gleanings in Bee Culture (Volume 20):

If you do not wish to have your name as informant mentioned in connection with the matter, nor any thing done about it at all, say so; at any rate, tell us where you know of a producer who is engaged in the mixing business, and we will keep it “under our hat” if you say so.

The idiom remains unchanged today and is used often, with most people understanding what you mean when you tell someone to “keep it under your hat.”

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