means A nut is a hard-shelled seed or fruit you crack open to eat, or a small metal block with a threaded hole that screws onto a bolt to hold things together — and, informally, a person who's eccentric, obsessed, or slightly unhinged.

from From Old English 'hnutu,' the woody-shelled seed, with cousins across the Germanic family (Old Norse 'hnot,' Dutch 'noot,' German 'Nuss') and likely deeper roots in a Proto-Indo-European source also tied to Latin 'nux.' The fastener sense came later by resemblance — a small, hard, tightly-clenched lump — and the 'crazy person' sense grew from the slang use of 'nut' for the head (as in 'off his nut'), the idea being that something had come loose inside it.