'ratchet' originates from Middle English, probably from Old French rachet/ratchet (a diminutive form), itself of uncertain origin; the term was used for a toothed device from the early modern period.

'ratchet' changed from Old French forms such as 'rachet' into Middle English 'ratchet' and later stabilized as the modern English 'ratchet', carrying its technical sense into mechanical and metaphorical uses.

Initially, it meant 'a toothed wheel or device that allows movement in one direction'; over time it retained that technical meaning while extending metaphorically to mean 'to increase stepwise' and, separately in US slang, to describe rough or vulgar behavior/appearance.