Source : Webster's Revised Unabridged Dictionary (1913)

Infernally \In*fer"nal*ly\, adv.
   In an infernal manner; diabolically. ``Infernally false.''
   --Bp. Hacket.

Source : WordNet®

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