thinking cap

a state of mind marked by reflection or concentration: If we put on our thinking caps, we may come up with the answer.

Origin of thinking cap

First recorded in 1870–75

I will try to tell one if you will let me put on my thinking-cap for a moment.

Blue Robin, the Girl Pioneer |Rena I. Halsey
"And this head of mine would have to put on its thinking-cap," he pursued jovially.

The Leavenworth Case |Anna Katherine Green
There was nothing for it but to bow to the inevitable, break my journey, and put my thinking-cap on.

The Chronicles of a Gay Gordon |Jos Maria Gordon
In his mechanical work his inventive turn of mind always caused him to put on his thinking-cap when he faced an obstacle.

Airship Andy |Frank V. Webster

Now here's my idea that I evolved with my thinking-cap—I mean night-cap—on!

Glory and the Other Girl |Annie Hamilton Donnell

thinking cap

see put on one's thinking cap.