bitter (adj.)
Old English biter "bitter, sharp, cutting; angry, embittered; cruel," from Proto-Germanic *bitras- (cognates: Old Saxon bittar, Old Norse bitr, Dutch bitter, Old High German bittar, German bitter, Gothic baitrs "bitter"), from PIE root *bheid- "to split" (cognates: 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.