1.
a. An indehiscent, hard-shelled, one-loculated, one-seeded fruit, such as an acorn or a hazelnut. b. A seed borne within a fruit having a hard shell, as in the peanut, almond, or walnut. c. The kernel of any of these.
2. Slang. a. A crazy or eccentric person. b. An enthusiast; a buff: a movie nut.
3. Informal. A difficult endeavor or problem: Painting the closet was a tough nut to crack.
4. Slang. The human head.
5. Music. a. A ridge of wood at the top of the fingerboard or neck of a stringed instrument, over which the strings pass. b. A device at the lower end of the bow for a stringed instrument, used for tightening the hairs.
6. A small block of metal or wood with a central, threaded hole that is designed to fit around and secure a bolt or screw.
7. Slang. a. The cost of launching a business venture. b. The operating expenses of a theater, theatrical production, or similar enterprise: "The [theater] has simply failed to attract enough paying customers per week to meet its nut" (Variety).
8. nuts Vulgar Slang. The testicles.