From The Collaborative International Dictionary of English v.0.48 [gcide]:
fiend \fiend\ (f[=e]nd), n. [OE. fend, find, fiend, feond,
fiend, foe, AS. fe['o]nd; akin to OS. f[imac]ond, D. vijand
enemy, OHG. f[imac]ant, G. feind, Icel. fj[=a]nd, Sw. & Dan.
fiende, Goth. fijands; orig. p. pr. of a verb meaning to
hate, AS. fe['o]n, fe['o]gan, OHG. f[imac]?n, Goth. fijan,
Skr. p[imac]y to scorn; prob. akin to E. feud a quarrel.
[root]81. Cf. Foe, Friend.]
An implacable or malicious foe; one who is diabolically
wicked or cruel; an infernal being; -- applied specifically
to the devil or a demon.
[1913 Webster]
Into this wild abyss the wary fiend
Stood on the brink of Hell and looked a while.
--Milton.
[1913 Webster]
O woman! woman! when to ill thy mind
Is bent, all hell contains no fouler fiend. --Pope.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
fiend
n 1: a cruel wicked and inhuman person [syn: monster, fiend,
devil, demon, ogre]
2: an evil supernatural being [syn: devil, fiend, demon,
daemon, daimon]
3: 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"--Winston Churchill [syn: fanatic, fiend]