Brain  —n. 1 organ of soft nervous tissue in the skull of vertebrates, the centre of sensation and of intellectual and nervous activity. 2 a colloq. Intelligent person. B (often in pl.) Intelligence. 3 (usu. In pl.; prec. By the) colloq. Cleverest person in a group; mastermind. 4 electronic device functioning like a brain. —v. 1 dash out the brains of. 2 colloq. Strike hard on the head.  on the brain colloq. Obsessively in one's thoughts. [old english]