Old English idel "empty, void; vain; worthless, useless," from Proto-West Germanic *idla- (source also of Old Saxon idal, Old Frisian idel "empty, worthless," Old Dutch idil, Old High German ital, German eitel "vain, useless, mere, pure"), a word of unknown origin.
Subsequent developments are peculiar to English: sense "not employed, not doing work" was in late Old English in reference to persons; from 1520s of things; from 1805 of machinery. Meaning "lazy, slothful" is from c. 1300. In Elizabethan English it also could mean "foolish, delirious, wandering in the mind." Idle threats preserves original sense.