grey matter

noun
  1. 1

    Brains; a person's ability to think.

  2. 2

    Literal brains as a general substance.

    โ€œ"He fell from the 5th floor. There is grey matter on the path behind his head. I'm afraid he's dead."โ€

  3. 3

    A collection of cell bodies and (usually) dendritic connections, in contrast to white matter.

Source: https://en.wiktionary.org/wiki/grey%20matter