late 15c., "make vain or worthless" (trans.), from idle (adj.). Meaning "spend or waste (time)" is from 1650s. Meaning "cause to be idle" is from 1788. Intrans. sense of "run slowly and steadily without transmitting power" (as a motor) first recorded 1916. Related: Idled; idling. As a noun, 1630s of persons, 1939 of an engine setting.