A pin or wedge inserted through a slot to hold machine parts together.
The chains are secured to each end of the cast-iron arches by cotters. 1949 November and December, “Notes and News: The High Level Bridge, Newcastle”, in Railway Magazine, page 407:
ru шпо́нка (pin to hold parts together) · ru клин (pin to hold parts together) · fi sokka (cotter pin) · bg щифт (pin to hold parts together) · fi tappi (pin to hold parts together) · fi kiila (pin to hold parts together) · ru суха́рь (pin to hold parts together) · ru чека́ (pin to hold parts together) · hu sasszeg (cotter pin) · bg шпонка (pin to hold parts together)
She found Esco by the barn. He was trying to cotter a cart-wheel with a peg he had whittled from a locust branch, driving it in with a hand sledge 1997, Charles Frazier, chapter 2, in Cold Mountain, London: Hodder and Stoughton, page 39:
From Middle English cotter, cotere, kottere, koter, cotier, equivalent to cot (“cottage”) + -er, from Old English cot. Compare Old French coter, cotier.
Meanings
(historical) A peasant who performed labour in exchange for the right to live in a cottage.
The toil-worn Cotter frae his labour goes,— / This night his weekly moil is at an end,— / Collects his spades, his mattocks, and his hoes, […] 1786, Robert Burns, The Cotter's Saturday Night: