castellated nut

noun
  1. a tall lock nut, having on its outer face radial slits allowing for insertion of a cotter pin or wire in both the nut and a hole in its bolt, so as to prevent the nut from coming loose.

Origin of castellated nut

First recorded in 1900–05
Also called castle nut.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018