There are many sort of nuts - peanuts, cashews, pistachios, Brazil nuts and more.
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.
1998, Brian Hingley, Furniture Repair & Refinishing - Page 95
As the bolt tightens into the nut, it pulls the tenon on the side rail into the mortise in the bedpost and locks them together. There are also some European beds that reverse the bolt and nut by setting the nut into the bedpost with the bolt inserted into a slotted area in the side of the rail.
1971, Hunter S. Thompson, Fear and Loathing in Las Vegas, Harper Perennial (2005), page 11:
My attorney was waiting in a bar around the corner. “This won't make the nut,” he said, “unless we have unlimited credit.”
(US,slang) A stash of money owned by an extremely rich investor, sufficient to sustain a high level of consumption if all other money is lost.
(music, lutherie) On string instruments such as guitars and violins, the small piece at the peghead end of the fingerboard that holds the strings at the proper spacing and, in most cases, the proper height.
(typographyslang)En, a unit of measurement equal to half of the height of the type in use.
1914, "Saki", ‘The Dreamer’, Beasts and Superbeasts, Penguin 2000 (Complete Short Stories), p. 323:
‘You are not going to be what they call a Nut, are you?’ she inquired with some anxiety, partly with the idea that a Nut would be an extravagance which her sister's small household would scarcely be justified in incurring [...].
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1999, Nik Cohn, Yes we have no: adventures in the other England
One night, we were fumbling each other out by the toilets when a Rocker in full leathers came out of the Gents and, without breaking stride or saying a word, nutted me square between the eyes. I went down as though shot...