Keep it under your hat is an interesting idiom that has a somewhat convoluted origin. We will examine the meaning of the phrase keep it under your hat, where it most probably came from and some examples of its use in sentences.
Keep it under your hat is an admonishment to keep something secret. Over the years, the idiom keep it under your hat seems to have undergone a slight shift in meaning. In the mid-1800s the term was used in Britain as an admonishment to keep something in your head, to leave something in your imagination and not bring it to fruition. The oldest example of this use of the phrase is in The History of Pendennis by William Makepeace Thackeray, published in 1848. The idiom traveled to America and underwent a shift in meaning by the 1890s to its current definition, to keep something secret. There are some who believe that this shift in meaning might be attributable to Abraham Lincoln’s habit of secreting important papers inside the lining of his stovepipe hat. In fact, Lincoln often referred to his hat as his office. There is an origin story that states that keep it under your hat dates back to a time when archers kept their bowstrings under their hats in order to keep them dry. While this is true, the first examples of the idiom keep it under your hat are not found until centuries later.
Examples
He says: ‘Keep it under your hat but I’ve got a horse you might like to look at for the Derby’.” (The Telegraph)
So if greasy pastries don’t take your fancy, keep it under your hat as you don’t want to anger a hungry Geordie. (The Shields Gazette)
Learn to keep it under your hat until the appropriate time, and all will be well. (The Business Insider)