n.1. a. An indehiscent fruit having a single seed enclosed in a hard shell, such as an acorn or hazelnut.
b. The usually edible seed of such a fruit.
c. Any of various other usually edible seeds enclosed in a hard covering such as a seed coat or the stone of a drupe, as in a pine nut, peanut, almond, or walnut.
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. Vulgar Slang A testicle.