Definition of “nut” - Learner’s Dictionary

nut

noun [ C ] uk us /nʌt/
FOOD

B2 the dry fruit of some trees that grows in a hard shell, and can often be eaten:

METAL

a piece of metal with a hole in it through which you put a bolt (= metal pin) to hold pieces of wood or metal together

KEEN informal

a person who is keen on a particular subject or hobby:

She's a real sports nut.

(Definition of “nut” from the Cambridge Learner's Dictionary © Cambridge University Press)