rack one's brains

noun

1. the mass of nerve tissue in the cranium of vertebrate animals, an enlarged extension of the spinal cord: it is the main part of the nervous system, the center of thought, and the organ that perceives sensory impulses and regulates motor impulses: it is made up of gray matter (the outer cortex of nerve cells) and white matter (the inner mass of nerve fibers)
2. a comparable organ in invertebrate animals
3. 1. intelligence; mental ability
   2. Informal a person of great intelligence
4. Informal the main organizer or planner of a group activity; chief controller or director

transitive verb

1. to dash out the brains of
2. Slang to hit hard on the head