It is difficult for non-combatants to understand how the bitterest of enemies can become friends.
Few [who saw the play] realized that this was the bitterest and most meaningful lesson.
Of course, Don ends up betraying Ms. Farrell in one of the bitterest scenes in Mad Men history.
Old English biter "bitter, sharp, cutting; angry, embittered; cruel," from Proto-Germanic *bitras- (cf. Old Saxon bittar, Old Norse bitr, Dutch bitter, Old High German bittar, German bitter, Gothic baitrs "bitter"), from PIE root *bheid- "to split" (cf. 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.
Bitterness is symbolical of affliction, misery, and servitude (Ex. 1:14; Ruth 1:20; Jer. 9:15). The Chaldeans are called the "bitter and hasty nation" (Hab. 1:6). The "gall of bitterness" expresses a state of great wickedness (Acts 8:23). A "root of bitterness" is a wicked person or a dangerous sin (Heb. 12:15). The Passover was to be eaten with "bitter herbs" (Ex. 12:8; Num. 9:11). The kind of herbs so designated is not known. Probably they were any bitter herbs obtainable at the place and time when the Passover was celebrated. They represented the severity of the servitude under which the people groaned; and have been regarded also as typical of the sufferings of Christ.