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).
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<a href="https://www.etymonline.com/word/assertiveness">Etymology of assertiveness by etymonline</a>
Harper, D. (n.d.). Etymology of assertiveness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/assertiveness