Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

hellishly
     adv : extremely; "infernally clever"; "hellishly dangerous" [syn:
           {infernally}]