Other forms: fiends
A fiend is a cruel or wicked person, or a keen enthusiast. A cruel fiend, a fresh-air fiend, a fiend for detail.
The two senses could not be further apart and both are common — the enthusiast sense is affectionate and always takes for or a compound. The Old English word simply meant an enemy, and it took on the devil later. Fiendish means both wicked and very difficult.
1nounfiendA cruel or wicked person.
“The fiend who poisoned the village well was never caught.”
2nounkeenA keen enthusiast.
“My grandmother is a crossword fiend and finishes one every morning before breakfast.”
fiend is a person.
Old English feond means an enemy, the present participle of a verb meaning to hate. Friend was formed the same way from a verb meaning to love.
Proto-Indo-European
*peh₁-
Proto-Germanic
*fijandz
Proto-West Germanic
*fijand
Old English
fēond — enemy
Middle English
fend
English, today
fiend
Built from
From Middle English fend, feend (“enemy; demon”), from Old English fēond (“enemy”), Proto-West Germanic fijand, from Proto-Germanic fijandz. 2 Cognate with Scots fient (“fiend”), stq Fäind (“enemy, fiend, foe”), cim faint (“enemy, fiend”), Dutch vijand (“enemy”), German Feind (“enemy, fiend, foe”), wym faeind, fajnd (“enemy”), Yiddish פֿײַנד, פֿײַנט (“enemy”), Danish fjende (“adversary, enemy, foe”), Icelandic fjandi (“enemy; fiend, demon, devil”), Norwegian Bokmål -, Norwegian Nynorsk - and Swedish fiende (“enemy”), Old Norse fjándi (“enemy; devil”), Gothic 𐍆𐌹𐌰𐌽𐌳𐍃, 𐍆𐌹𐌾𐌰𐌽𐌳𐍃 (“enemy, foe”). The Old Norse and Gothic terms are present participles of the corresponding verbs fjá/𐍆𐌹𐌾𐌰𐌽 (“to hate”), from Proto-Indo-European peh₁- (“to hate”) (compare Sanskrit पीयति (“(he) reviles”)).
Etymology from Wiktionary, CC BY-SA