idleness (n.)

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.

word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).