Why is the ninth month called September?
c.1200, "fighting equipment," probably from Old Norse gervi "apparel," related to gerr "ready," and gerva "make ready," from Proto-Germanic *garwin- (cf. Old English gearwe "clothing, equipment, ornament;" Old High German garawi "clothing, dress," garawen "to make ready;" German gerben "to tan"). Meaning "toothed wheel in machinery" first attested 1520s. Slang for "male sex organs" from 1670s. British adjective slang sense of "stylish, excellent" first recorded 1951, from earlier that's the gear, expression of approval, 1925.
c.1200, "to equip oneself for fighting; dress," probably from gear (n.). Related: Geared; gearing.
A wheel with teeth around its rim that mesh with the teeth of another wheel to transmit motion. Gears are used to transmit power (as in a car transmission) or change the direction of motion in a mechanism (as in a differential axle). Fixed ratios of speed in various parts of a machine is often established by the arrangement of gears.