foe (n.) Look up foe at Dictionary.com
Old English gefa "foe, enemy, adversary in a blood feud" (the prefix denotes "mutuality"), from fah "at feud, hostile," from Proto-Germanic *fakhaz (cognates: Old High German fehan "to hate," Gothic faih "deception"), probably from PIE root *peig- (2) "evil-minded, treacherous, hostile" (cognates: Sanskrit pisunah "malicious," picacah "demon;" Greek pikros "bitter;" Latin piget "it irks, troubles, displeases," piger "reluctant, lazy;" Lithuanian piktas "wicked, angry," pekti "to blame"). Weaker sense of "adversary" is first recorded c.1600.