fiend /fiːnd/ BrE AmE noun [countable]
[ Language: Old English ; Origin: feond 'enemy' ]
1 . a very cruel, evil, or violent person:
a heartless fiend
Both of them were fighting like fiends.
a sex/rape fiend (=one who commits very unpleasant sex attacks)
2 . television/sports/fresh-air etc fiend someone who likes watching television, doing sports etc a lot, or more than is normal SYN fanatic
3 . drug/dope/cocaine etc fiend someone who takes a lot of illegal drugs
4 . an evil spirit