gear-shifting mechanism on a bicycle that shifts the drive train from one sprocket to another.
Origin: 1945-50; <French dérailleur literally, a device causing disengagement or derailing, equivalent to déraill (er) to derail +-eur -eur(It's kind of fun to look up words in the dictionary every now and then. Try the paper kind, too.)
What simple tasks do you easily forget?
*Dictionary spelling and origin borrowed from dictionary.com