Nut (noun)

1. A hard-shelled seed.
2. A simple machine consisting of a screw that turns in a socket and is used especially for holding things together.
3. A crazy or eccentric person.
4. Testicles.

Origin:

Middle english, from old english hnutu; akin to old high german hnuz nut and perhaps to old english cnāwan to know.

Examples:

1. She cracked open the nut with a nutcracker.
2. The bolt and nut held the board securely to the fence.
3. Some of his friends called him a nut, but he took it as a compliment.
4. He clutched at his nut as the foul ball rushed towards him.
5. The squirrel buried the nut in the ground.