People who are extremely evil or wicked; also used playfully to mean someone who is obsessed with something like a 'coffee fiend.'
From Old English 'feond' meaning enemy or foe, from Proto-Germanic root meaning to hate or be hostile.
The word 'fiend' shifted from just meaning 'enemy in battle' to 'evil being'—showing how once you fight someone, you start seeing them as morally corrupt, which is a dangerous pattern in human conflict psychology.
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