fiend (n.) Look up fiend at Dictionary.com
Old English feond "enemy, foe," 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" (source also of Gothic faian "to blame;" see passion).

As spelling suggests, the word originally was the opposite of friend, but it began to be used in Old English for "Satan" (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.