Friend and fiend have identical formations: They are both in origin present participles used as nouns, Old English frēond (also frīend ) for friend, and fēond (also fīend ) for fiend. The two nouns even occur together in Old English alliterative verse: Se fēond and se frēond “the fiend and the friend.”
Frēond “friend, close acquaintance” has many cognates in Germanic: Old Frisian friūnd, Old Dutch friunt, Old High German friunt, German Freund, Gothic frijonds. Frēond comes from the Old English verb frēogan (also frēon ) “to love, free, set free,” and is a derivative of the Germanic root fri-, frī- (and suffixed form frija- ), which is also the source of English free (the progression of senses is “beloved,” then “one of the loved ones,” then “one not a slave, free”).
Old English fēond originally meant “enemy, foe” (and so was the opposite of friend ), and especially in Old English poetry, “Satan, the Devil” (in Beowulf the devil is referred to as fēond moncynnes “the enemy of mankind”). Fēond has many cognates in Germanic: Old Frisian fiand, Dutch vijand, German Feind, all meaning “enemy.” Fēond comes from the Old English verb fēogan “to hate,” from a Germanic root fī - (from a very complicated Proto-Indo-European root pē-, pēi-, pī- “to hurt, harm”).
Etymologically speaking, then, friend and fiend are acquaintances, and not relatives.

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