What does "brain" mean?

Additional senses

1. 2.The organ or seat of intellect; hence, the understanding. " My brain is too dull." Sir W. Scott. Note: In this sense, often used in the plural.
2. 3.The affections; fancy; imagination. [R.] Shak. To have on the brain, to have constantly in one's thoughts, as a sort of monomania. [Low] Brain box or case, the bony on cartilaginous case inclosing the brain. -- Brain coral, Brain stone coral (Zoöl), a massive reef-building coral having the surface covered by ridges separated by furrows so as to resemble somewhat the surface of the brain, esp. such corals of the genera Mæandrina and Diploria. -- Brain fag (Med.), brain weariness. See Cerebropathy. -- Brain fever (Med.), fever in which the brain is specially affected; any acute cerebral affection attended by fever. -- Brain sand, calcareous matter found in the pineal gland.
3. 4.To dash out the brains of; to kill by beating out the brains. Hence, Fig.: To destroy; to put an end to; to defeat. There thou mayst brain him. Shak. It was the swift celerity of the death . . . That brained my purpose. Shak.
4. 5.To conceive; to understand. [Obs.] brain not. Shak.