mid-13c., "harboring ill-will, enmity, or hostility," from Old French malicios "showing ill will, spiteful, wicked" (Modern French malicieux), from Latin malitiosus "wicked, malicious," from malitia "badness, ill will, spite," from malus "bad, unpleasant" (see mal-). In legal use (early 14c., Anglo-French), it means "characterized by malice prepense" (see malice).
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/maliciousness">Etymology of maliciousness by etymonline</a>
Harper, D. (n.d.). Etymology of maliciousness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/maliciousness