1530s "gloomy, of a sour temper, sullen and austere," from Latin morosus "morose, peevish, hypercritical, fastidious," from mos (genitive moris) "habit, custom" (see moral (adj.)). In English, manners by itself means "(good) manners," but here the implication in Latin is "(bad) manners."
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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Harper, D. (n.d.). Etymology of moroseness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/moroseness