fiend (n.) Look up fiend at Dictionary.com
Old English feond "enemy, foe, adversary," originally present participle of feogan "to hate," from Proto-Germanic *fijand- "hating, hostile" (cognates: Old Frisian fiand "enemy," Old Saxon fiond, Middle Dutch viant, Dutch vijand "enemy," Old Norse fjandi, Old High German fiant, Gothic fijands), from suffixed form of PIE root *pe(i)- "to hurt" (cognates: Sanskrit pijati "reviles, scorns," Greek pema "suffering, misery, woe," Gothic faian "to blame," and possibly Latin pati "to suffer, endure"). According to Watkins, not allied to foe and feud (n.).

As spelling suggests, the word originally was the opposite of friend and described any hostile enemy (male and female, with abstract noun form feondscipe "fiendship"), but it began to be used in late Old English for "the Devil, Satan" (literally "adversary") as the "enemy of mankind," which shifted its sense to "diabolical person" (early 13c.). The old sense of the word devolved to foe, then to the imported word enemy. For spelling with -ie- see field. Meaning "devotee (of whatever is indicated)," as in dope fiend, is from 1865.