noun

1. an evil spirit; devil
2. an inhumanly wicked or cruel person
3. ☆ Informal
   1. a person addicted to some activity, habit, etc. a fresh-air fiend
   2. a person who is excellent at some activity a fiend at tennis

Etymology: ME fend, feend < OE feond, lit., the one hating, orig. prp. < base of feogan, to hate, akin to Goth fijands < fijan, to hate < IE base *pē(i)-, to harm