linchpin or lynchpin [linch-pin] (previously 03-11-13)
noun
1. a pin inserted through the end of an axletree to keep the wheel on.
2. something that holds the various elements of a complicated structure together: The monarchy was the linchpin of the nation's traditions and society.
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Origin: 1350-1400; unexplained alteration of Middle English lynspin, equivalent to lyns, Old English lynis axle-pin (cognate with German Lünse) + pin pin
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