Keep it under your hat

Meaning

Keep it secret.

Origin

On first hearing this seems a rather strange phrase. Why should people put anything under their hats and, even if they were to, why would that be associated with secrecy? The speculation is that putting an item under one's hat would be a way of hiding it. Such trickery is recorded, as in the collection of stories, published as The Adventurer, 1793:

"By a sudden stroke of conjuration, a great quantity of gold might be conveyed under his hat."

The most commonly repeated speculation on the 'hiding under one's hat' theory of the origin of this expression is that English archers in mediaeval times used to store spare bowstrings under their hats to keep them dry. Let's just get that out of the way. Firstly, keeping dry isn't keeping secret, so even if archers did store strings that way, and there's no evidence that they did, where is the connection to the phrase's meaning? Secondly, and it would have been kinder to put this first as it entirely dismisses the archer tale, the phrase isn't known in English until the 19th century - so much for a mediaeval origin.

It is much more likely that there's no direct link to hats at all and that 'keep it under your hat' just meant 'keep it in your head'. That's the meaning alluded to in the earliest citations of the phrase in print. The oldest of such that I can find is in Charlotte Mary Yonge's novel, Nuttie's Father, 1885.

"Alice Egremont's loving and unsuspecting heart was so entirely closed against evil thoughts of her husband... while Nuttie, being essentially of a far more shrewd and less confiding nature, was taking in all these revelations... It was all under her hat, however, and the elder ladies never thought of her, Alice bringing back the conversation to Mrs. Houghton herself."

The same meaning is evident in Anthony Trollope's What I Remember, 1887–89:

"The man whose estate lies under his hat need never tremble before the frowns of fortune."