Etymology
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foe (n.)

Old English gefea, gefa "foe, enemy, adversary in a blood feud" (the prefix denotes "mutuality"), from adjective fah "at feud, hostile," also "guilty, criminal," from Proto-Germanic *faihaz (source also of Old High German fehan "to hate," Gothic faih "deception"), perhaps from the same PIE source that yielded Sanskrit pisunah "malicious," picacah "demon;" Lithuanian piktas "wicked, angry," peikti "to blame." Weaker sense of "adversary" is first recorded c. 1600.

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foe-man (n.)
also foeman, "active enemy," late Old English fah-man; see foe + man (n.).
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fey (adj.)

"of excitement that presages death," from Old English fæge "doomed to die, fated, destined," also "timid, feeble;" and/or from Old Norse feigr, both from Proto-Germanic *faigjo- (source also of Old Saxon fegi, Old Frisian fai, Middle Dutch vege, Middle High German veige "doomed," also "timid," German feige "cowardly"), from the same source as foe. Preserved in Scottish. Sense of "displaying unearthly qualities" and "disordered in the mind (like one about to die)" led to modern ironic sense of "affected."

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feud (n.)
c. 1300, fede "enmity, hatred, hostility," northern English and Scottish, ultimately (via an unrecorded Old English word or Old French fede, faide "war, raid, hostility, hatred, enmity, feud, (legal) vengeance," which is from Germanic) from Proto-Germanic *faihitho (compare Old High German fehida "contention, quarrel, feud"), noun of state from adjective *faiho- (source also of Old English fæhð "enmity," fah "hostile;" German Fehde "feud;" Old Frisian feithe "enmity"). Perhaps from the same PIE source as foe. Sense of "vendetta" is early 15c. Alteration of spelling in 16c. is unexplained. Meaning "state of hostility between families or clans" is from 1580s.
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fickle (adj.)
c. 1200, "false, treacherous, deceptive, deceitful, crafty" (obsolete), probably from Old English ficol "deceitful, cunning, tricky," related to befician "deceive," and to facen "deceit, treachery; blemish, fault." Common Germanic (compare Old Saxon fekan "deceit," Old High German feihhan "deceit, fraud, treachery"), from the same source as foe.

Sense of "changeable, inconstant, unstable" is from c. 1300 (especially of Fortune and women). Related: Fickleness. Fickly (c. 1300) is rare or obsolete. Also with a verb form in Middle English, fikelen "to deceive, flatter," later "to puzzle, perplex," which survived long enough in Northern dialects to get into Scott's novels. Fikel-tonge (late 14c.) was an allegorical or character name for "one who speaks falsehoods."
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password (n.)

"secret word appointed as a sign to distinguish friend from foe," 1798, from pass (v.) + word (n.).

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arch-fiend (n.)

1667, from arch (adj.) + fiend (n.). Originally and typically Satan (arch-foe "Satan" is from 1610s).

So stretcht out huge in length the Arch-fiend lay. ["Paradise Lost," 1667]
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Philistine 

one of the Old Testament people of coastal Palestine who made war on the Israelites, early 14c., from Old French Philistin, from Late Latin Philistinus, from Late Greek Philistinoi (plural), from Hebrew P'lishtim, "people of P'lesheth" ("Philistia"); compare Akkadian Palastu, Egyptian Palusata; the word probably is the people's name for themselves. Hence, "a heathen enemy, an unfeeling foe" (c. 1600).

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ofay (n.)
African-American vernacular, "white person," 1925, of unknown origin. If, as is sometimes claimed, it derives from an African word, none corresponding to it has been found. Perhaps the most plausible speculation is Yoruba ófé "to disappear" (as from a powerful enemy), with the sense transferred from the word of self-protection to the source of the threat. OED regards the main alternative theory, that it is pig Latin for foe, to be no more than an "implausible guess." Sometimes shortened to fay (1927).
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quarter (n.2)

"indulgence or mercy shown to a vanquished foe; exemption from being immediately killed upon being defeated in battle or armed contest," 1610s, presumably from quarter (n.1) or its French equivalent quartier in this meaning, but OED writes that "the precise origin of this sense is obscure ...." It suggests quarter in a now-obsolete sense of "relations with, or conduct towards, another" (attested from 1640s), or possibly quarter in the sense of "place of stay or residence" (compare quarters), on the notion of sending the vanquished to an assigned place until his fate is decided. German quartier, Swedish quarter, Danish kvarteer, etc. probably are from French.

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