rack brain

To struggle to recall or think of something. I've been racking my brain, but I still can't remember what Lydia's husband's name is. He racked his brains all weekend trying to think of a solution to the problem.

Fig. to try very hard to think of something. I racked my brains all afternoon, but couldn't remember where I put the book. Don't waste any more time racking your brain. Go borrow the book from the library.

To think long and hard: I racked my brain for hours trying to recall her name.

To strain to remember something or discover a solution. The rack here is the medieval instrument of torture on which the victim’s body was stretched until it broke. The idea is old; “we break our brains for naught” comes from 1530. The word “rack” came into use about 150 years later.