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.

Words nearby thinking cap

First recorded in 1870–75

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

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.