idle: [OE] ‘Lazy’ is only a secondary meaning of idle. It originally meant ‘useless, worthless’ (as in ‘idle threats’), and the sense ‘lazy’ did not develop until the 13th century (the Old English words for ‘lazy’ were slow and slack). Idle is shared by other West Germanic languages, and its relatives (German eitel ‘vain, futile’ and Dutch ijdel ‘vain, useless, conceited’) point up its original English meaning, but it is not known what its ultimate origins are.
Old English idel "empty, void; vain; worthless, useless; not employed," common West Germanic (cognates: Old Saxon idal, Old Frisian idel "empty, worthless," Old Dutch idil, Old High German ital, German eitel "vain, useless, mere, pure"), of unknown origin. Idle threats preserves original sense; meaning "lazy" is c. 1300.
late 15c., "make vain or worthless," from idle (adj.). Meaning "spend or waste (time)" is from 1650s. Meaning "cause to be idle" is from 1789. Sense of "running slowly and steadily without transmitting power" (as a motor) first recorded 1916. Related: Idled; idling.
