bitterness (n.)

Old English biter "having a harsh taste, sharp, cutting; angry, full of animosity; cruel," from Proto-Germanic *bitras- (source also of Old Saxon bittar, Old Norse bitr, Dutch bitter, Old High German bittar, German bitter, Gothic baitrs "bitter"), from suffixed form of PIE root *bheid- "to split" (source also of Old English bitan "to bite;" see bite (v.)). Evidently the meaning drifted in prehistoric times from "biting, of pungent taste," to "acrid-tasting." Used figuratively in Old English of states of mind and words. Related: Bitterly.

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).