[syn: demonic, diabolic, diabolical, fiendish, hellish, infernal, satanic, unholy]
The Collaborative International Dictionary of English v.0.48:
Hellish \Hell"ish\, a.
   Of or pertaining to hell; like hell; infernal; malignant;
   wicked; detestable; diabolical. "Hellish hate." --Milton. --
   Hell"ish*ly, adv. -- Hell"ish*ness, n.
   [1913 Webster]
WordNet (r) 3.0 (2006):
hellish
    adj 1: very unpleasant; "hellish weather"; "stop that god-awful
           racket" [syn: beastly, hellish, god-awful]
    2: extremely evil or cruel; expressive of cruelty or befitting
       hell; "something demonic in him--something that could be
       cruel"; "fires lit up a diabolic scene"; "diabolical
       sorcerers under the influence of devils"; "a fiendish
       despot"; "hellish torture"; "infernal instruments of war";
       "satanic cruelty"; "unholy grimaces" [syn: demonic,
       diabolic, diabolical, fiendish, hellish, infernal,
       satanic, unholy]
Moby Thesaurus II by Grady Ward, 1.0:
98 Moby Thesaurus words for "hellish":
   Acherontic, Draconian, Lethean, Mephistophelian, Plutonian,
   Plutonic, Stygian, Tartarean, anarchic, angry, animal,
   anthropophagous, atrocious, barbaric, barbarous, beastly, bestial,
   bloodthirsty, bloody, bloody-minded, blustering, blusterous,
   blustery, brutal, brutalized, brute, brutish, cannibalistic,
   chaotic, chthonian, chthonic, cruel, cruel-hearted, cursed,
   damnable, demoniac, demoniacal, demonic, demonish, demonlike,
   devil-like, devilish, diabolic, execrable, fell, feral, ferocious,
   fiendish, fiendlike, fierce, frantic, frenzied, furious, ghoulish,
   hellborn, infernal, infuriate, inhuman, inhumane, insensate, mad,
   mindless, murderous, ogreish, orgasmic, orgastic, pandemoniac,
   pandemonic, purgatorial, raging, ravening, raving, rip-roaring,
   ruthless, sadistic, sanguinary, sanguineous, satanic, savage,
   sharkish, slavering, storming, stormy, subhuman, sulfurous,
   tempestuous, troublous, truculent, tumultuous, turbulent,
   unchristian, uncivilized, ungodly, unhuman, uproarious, vicious,
   wild, wolfish