Bitter

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Old English biter, of Germanic origin; related to Dutch and German bitter, and probably to bite.

wiktionary

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From Middle English bitter, bittre, from Old English bitter, biter(“bitter”), from Proto-West Germanic *bit(t)r, from Proto-Germanic *bitraz(“bitter”), equivalent to bite +‎ -er(adjectival suffix). Compare Saterland Frisian bitter(“bitter”), West Frisian bitter(“bitter”), Dutch bitter(“bitter”), Low German bitter(“bitter”), German bitter(“bitter”), Swedish bitter(“bitter”), Icelandic bitur(“bitter”).

bit +‎  -er

etymonline

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bitter (adj.)

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.