Thinking cap

Put on one’s thinking cap is a metaphor for giving something or other due thought or consideration. In this form, it dates from the early 19th century and the earliest citation for it, according to Merriam-Webster, is 1827. Therefore, it appears to be of American origin, but before this, from at least the early 1600s, it was known in Britain as a ‘considering cap’ but this usage has now become obsolete. The metaphor derives from the allusion that different caps fulfil different functions, waterproof caps keep the rain off etc.