Gray-matter

noun
1.
Anatomy. nerve tissue, especially of the brain and spinal cord, that contains fibers and nerve cell bodies and is dark reddish-gray.
Compare .
2.
Informal. brains or intellect.

gray matter n.
Brownish-gray nerve tissue, especially of the brain and spinal cord, composed of nerve cell bodies and their dendrites and some supportive tissue. Also called gray substance, substantia grisea.
gray matter
The brownish-gray tissue of the vertebrate brain and spinal cord, made up chiefly of the cell bodies and dendrites of neurons. Compare white matter.

noun phrase

Intelligence; brains, smarts (1899+)
Brains, intellect, as in If you’d only use your gray matter, you’d see the answer in a minute. This expression refers to actual brain tissue that is gray in color. Agatha Christie’s fictional detective, Hercule Poirot, constantly alludes to using the little gray cells for solving a crime. [ Late 1800s ]