: to think very hard in order to try to remember something, solve a problem, etc. I've been racking my brain, but I can't remember his name.

Is it wracking my brain or racking my brain?

The spelling 'rack' is now used in all senses except for the seaweed called wrack. So it's "rack and ruin," … "racking my brains," and so on. Some other usage guides provide a way of dealing with this question that has a certain brutal charm: just stop using the word wrack.

Why do we say rack my brain?

Meaning: To think very hard to find an answer. If you rack your brains, you strain mentally to recall or to understand something. The rack was a mediaeval torture device where the victim was tied to the rack by his arms and legs, which were then practically torn from their bodies.