inertness (n.)

1640s, "without inherent force, having no power to act or respond," from French inerte (16c.) or directly from Latin inertem (nominative iners) "unskilled, incompetent; inactive, helpless, weak, sluggish; worthless," used of stagnant fluids, uncultivated pastures, expressionless eyes. It is a compound of in- "without, not, opposite of" (see in- (1)) + ars (genitive artis) "skill" (see art (n.)). In chemistry, "having no active properties, neutral" (1800), specifically from 1885 of certain chemically inactive, colorless, odorless gases. Of persons or creatures, "indisposed or unable to move or act," from 1774.

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).