The story
Picture pulling on a cap and feeling your brain switch on. That is the playful idea behind the thinking cap, and it is older than it sounds.
The first version was the considering cap. In 1605, the actor and comic writer Robert Armin, a clown in Shakespeare’s company, published a book of fool stories called Foole vpon foole. In it, “The Cobler puts off his considering cap” before he answers a question.
Children met it too. Little Goody Two-Shoes, a famous 1765 storybook, has a chapter on “the whole History of the Considering Cap,” and a later edition describes one “almost as large as a Grenadier’s,” a very tall hat indeed.
The thinking cap came later. The earliest one found so far is in the Western Carolinian, a newspaper in Salisbury, North Carolina, in October 1821: “We advise the editor to put his thinking-cap on, before he hazards another such assertion.”
Some say the phrase recalls the cap a judge once put on to deliver a verdict. Word historians doubt it. The early uses have nothing to do with courtrooms. The cap was always imaginary, which is exactly why anyone can put one on.