Why is it called cotter pin?
"wedge-shaped piece or bolt which fits into a hole used in fastening or tightening," 1640s, of uncertain origin; perhaps a shortened form of cotterel, a dialectal word for "cotter pin or bolt, bracket to hang a pot over a fire" (1560s), itself of uncertain origin. Cotter-pin is attested by 1849. cotter (n.)
What are cotter rings used for?