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.
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.
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Definitions of idly from WordNet
in an idle manner;
this is what I always imagined myself doing in the south of France, sitting idly, drinking coffee, watching the people