Nut

1. A fastener; a piece of metal, usually square or hexagonal in shape, with a hole through it having machined internal threads, intended to be screwed onto a bolt or other threaded shaft. We needed three half-inch hex nuts for the machine.
2. A hard-shelled seed. We grow hazel nuts on our farm.
3. (slang) A crazy person. He drove his car like a nut.
4. An extreme enthusiast. She was a fashion nut.
5. (vulgarity, slang rarely used in the singular) A testicle. They kicked him in the nuts.