the.com/fiend

means Someone with an obsessive devotion to a particular thing — a sugar fiend, a crossword fiend — borrowed from the demon's reputation for single-minded hunger.

from From Old English 'feond,' meaning enemy or devil, a word related to German 'Feind' (enemy) and ultimately tied to the root for 'hate' — it's the dark twin of 'friend,' which comes from the root for 'love.' The word named Satan himself for centuries before it softened, somewhere in modern usage, into a term for anyone hopelessly hooked on something — dope fiends, fitness fiends — trading hellfire for mere compulsion.