[n] one of the evil spirits of traditional Jewish and Christian belief [n] someone extremely diligent or skillful; "he worked like a demon to finish the job on time"; "she's a demon at math" [n] a cruel wicked and inhuman person
daemon, daimon, devil, fiend, monster, ogre
addict, afreet, ape-man, atua, Baba Yaga, barghest, beast, beldam, berserk, berserker, bomber, brute, bug, cacodemon, collector, daemon, daeva, damned spirits, demonkind, demons, denizens of hell, devil, devil incarnate, dragon, dybbuk, eager beaver, ener