A pin or wedge inserted through a hole in a shaft to secure it, or historically, a peasant farmer who lived in a cottage and worked for wages.
The mechanical meaning likely comes from cutting (cot-) + -er (tool), while the agricultural term derives from cot (cottage) + -er (one who dwells). Both emerged in Middle English.
A cotter pin is so simple but brilliant—it's the unsung hero keeping your car's wheel attached, and one medieval cotter (the farmer) might have earned enough in a year to buy the metal for a hundred cotter pins!
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