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derailleur

[dih-rey-ler]
noun
  1. a gear-shifting mechanism on a bicycle that shifts the drive chain from one sprocket wheel to another.
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Origin of derailleur

1945–50; < French dérailleur literally, a device causing disengagement or derailing, equivalent to déraill(er) to derail + -eur -eur
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for derailleur

derailleur

noun
  1. a mechanism for changing gear on bicycles, consisting of a device that lifts the driving chain from one sprocket wheel to another of different size
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Word Origin

French dérailleur derailer
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for derailleur

n.

type of bicycle gear mechanism, 1930, from French dérailleur (1927), from dérailler "to go off the rails" (see derail).

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Online Etymology Dictionary, © 2010 Douglas Harper