n.1 a an evil spirit, a demon. b (prec. by the) the Devil.
2 a a very wicked or cruel person. b a person causing mischief or annoyance.
3 (with a qualifying word) sl. a devotee or addict (a fitness fiend).
4 something difficult or unpleasant.
Derivatives:
fiendish adj. fiendishly adv. fiendishness n. fiendlike adj.
Etymology: OE feond f. Gmc