"fiend" in the noun sense
1. monster, fiend, devil, demon, ogre
a cruel wicked and inhuman person
2. devil, fiend, demon, daemon, daimon
an evil supernatural being
3. fanatic, fiend
a person motivated by irrational enthusiasm (as for a cause
"A fanatic is one who can't change his mind and won't change the subject"
Source: WordNet® (An amazing lexical database of English)
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