linchpin (n.) Look up linchpin at Dictionary.com
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.