Etymology
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linchpin (n.)
also linch-pin, "peg that holds a wheel on an axle" (now mainly figurative), late 14c., a corruption of linspin, literally "axle-pin," from pin (n.) + from Middle English lins "axle," from Proto-Germanic *luniso (source also of Old Saxon lunisa, Middle Dutch lunse, Dutch luns, German Lünse), a word of uncertain origin.
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linch (n.)
Old English lynis "linchpin," now obsolete; see linchpin.
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