Nut  n. 1 a fruit consisting of a hard or tough shell around an edible kernel. B this kernel. 2 pod containing hard seeds. 3 small usu. Hexagonal flat piece of metal etc. With a threaded hole through it for screwing on the end of a bolt to secure it. 4 slang person's head. 5 slang crazy or eccentric person. 6 small lump (of coal etc.). 7 (in pl.) Coarse slang testicles. do one's nut slang be extremely angry. [old english]