The story
Physically descriptive via a metaphor; called thus because the slots in the nut — the castellations — have a fancied resemblance to the battlements of a castle.
Etymology from Wiktionary, CC BY-SA 4.0.
castellated nut
Today it means: A type of nut (internally threaded fastener) with castellations, typically to hold a retaining pin that is passed through a cross-drilled hole, such as a cotter pin. Full definition →