a demon
(the Fiend) the Devil; Satan
a person of great wickedness or cruelty
informal somebody greatly or excessively devoted to a specified activity or thing; a fanatic or devotee
a golf fiend
a fresh-air fiend
informal a person who uses immoderate quantities of something specified; an addict
a dope fiend
informal somebody remarkably clever at a specified activity; = wizard1 (2)
She's a fiend at arithmetic
[Old English fīend enemy]