late 15c., "done thoughtlessly, heedless, careless, indiscreet," from Latin inconsideratus "headstrong, unadvised," of persons, "thoughtless," literally "not properly considered," from in- "not, opposite of" (see in- (1)) + consideratus (see consider). Sense of "without regard for the feelings of others" is from 1842. Related: Inconsiderately.
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).
<a href="https://www.etymonline.com/word/inconsiderateness">Etymology of inconsiderateness by etymonline</a>
Harper, D. (n.d.). Etymology of inconsiderateness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/inconsiderateness