O.E. gefa "adversary in deadly feud," from fah "at feud, hostile," from P.Gmc. *fakhaz (cf. O.H.G. fehan "to hate," Goth. faih "deception"), probably from PIE base *peik- "evil-minded, treacherous, hostile" (cf. Skt. pisunah "malicious," picacah "demon;" Gk. pikros "bitter;" Lith. piktas "wicked, angry," pekti "to blame"). Weaker sense of "adversary" is first recorded 1607.
Fiend\, n. [OE. fend, find, fiend, feond, fiend, foe, AS. fe['o]nd; akin to OS. f[=i]ond, D. vijand enemy, OHG. f[=i]ant, G. feind, Icel. fj[=a]nd, Sw. & Dan. fiende, Goth. fijands; orig. p. pr. of a verb meaning to hate, AS. fe['o]n, fe['o]gan, OHG. f[=i]?n, Goth. fijan, Skr. p[=i]y to scorn; prob. akin to E. feud a quarrel. [root]81. Cf. Foe, Friend.] An implacable or malicious foe; one who is diabolically wicked or cruel; an infernal being; -- applied specifically to the devil or a demon. Into this wild abyss the wary fiend Stood on the brink of Hell and looked a while. --Milton. O woman! woman! when to ill thy mind Is bent, all hell contains no fouler fiend. --Pope.
Foe\ (f[=o]), n. [OE. fo, fa, AS. f[=a]h hostile; prob. akin to E. fiend. [root]81. See Fiend, and cf. Feud a quarrel.]1. One who entertains personal enmity, hatred, grudge, or malice, against another; an enemy. A man's foes shall be they of his own household. --Matt. x. 36 2. An enemy in war; a hostile army. 3. One who opposes on principle; an opponent; an adversary; an ill-wisher; as, a foe to religion. A foe to received doctrines. --I. Watts