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rack (one's) brain(s)
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.
See also: rack
rack one's brain(s)
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.
rack (one's) brainInformal
To think long and hard: I racked my brain for hours trying to recall her name.