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]