Source : Webster's Revised Unabridged Dictionary (1913)

Incredibly \In*cred"i*bly\, adv.
   In an incredible manner.

Source : WordNet®

incredibly
     adv : not easy to believe; "behind you the coastal hills plunge to
           the incredibly blue sea backed by the Turkish
           mountains" [syn: {improbably}, {implausibly}, {unbelievably}]
           [ant: {believably}, {credibly}]