fiend: [OE] Fiend seems originally to have meant ‘hated person’. It was formed in prehistoric times from the past participle of a Germanic verb meaning ‘hate’ (represented in historic times by, for example, Old English fēon, Old High German fiēn, and Gothic fijan). In Old English its meaning had progressed to ‘enemy’ (which is what its German relative feind still means). Then towards the end of the first millennium AD we see evidence of its being applied to the ‘enemy’ of mankind, the Devil. From there it was a short step to an ‘evil spirit’ in general, and hence to any ‘diabolically wicked person’. [fiend etymology, fiend origin, 英语词源]