Etymology
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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.
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bitter end (n.)

In lexicons of sea language going back to 1759, the bitter end is the part of a cable which is round about the bitts (the two great timbers used to belay cables) when the ship is at anchor (see bitt).

Bitter end of the Cable, the End which is wound about the Bitts. ["The News-Readers Pocket-Book: Or, a Military Dictionary," London, 1759]

So, when a cable is played out to the bitter end, there is no more left to play. The term began to be used c. 1835 in non-nautical use and with probable influence of or merger with bitter (adj.).

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bitters (n.)
bitter-tasting medicines generally, 1713, from bitter. Especially a liquor in which bitter herbs or roots are steeped, used medicinally.
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bitterness (n.)
Old English biternys "bitterness, grief;" see bitter + -ness. Figurative sense (of feelings, etc.) is attested earlier than literal sense (of taste), which will surprise no one who reads any amount of Anglo-Saxon literature.
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embitter (v.)

"make bitter," c. 1600, from em- (1) + bitter (adj.). Now rare in its literal sense; figurative meaning "affect with bitterness or unhappiness" is attested by 1630s. Related: Embittered; embitterment.

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bittersweet (adj.)
"uniting bitterness and sweetness," 1610s, from bitter (adj.) + sweet (adj.). Perhaps older, as the same word is used as a noun in Middle English (late 14c.) for drinks or experiences that are both bitter and sweet and especially in reference to a type of apple; later of woody nightshade (1560s). Greek had a similarly formed compound, glykypikros, literally "sweet-bitter."
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*bheid- 
Proto-Indo-European root meaning "to split," with derivatives in Germanic "referring to biting (hence also to eating and to hunting) and woodworking" [Watkins].

It forms all or part of: abet; bait (n.) "food used to attract prey;" bait (v.) "to torment, persecute;" bateau; beetle (n.1) "type of insect; bit (n.1) "small piece;" bite; bitter; bitter end; boat; boatswain; -fid; fissile; fission; fissure; giblets; pita; pizza; vent (n.).

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhinadmi "I cleave," Latin Latin findere "to split, cleave, separate, divide," Old High German bizzan "to bite," Old English bita "a piece bitten off, morsel," Old Norse beita "to hunt with dogs," beita "pasture, food."
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taunt (n.)
1520s, "bitter invective," probably from taunt (v.).
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morello (n.)

kind of bitter cherry with a dark red skin, 1640s, a word of uncertain origin, perhaps ultimately from Latin amarus "bitter." Earlier form was morell (1610s).

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alum (n.)
"whitish mineral salt used as an astringent, dye, etc.," late 14c., from Old French alum, alun, from Latin alumen "alum," literally "bitter salt," cognate with Greek aludoimos "bitter" and perhaps with English ale. The Slavic words are said to be from Germanic.
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