The Collaborative International Dictionary of English v.0.48:

Incredibly \In*cred"i*bly\, adv.
   In an incredible manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

incredibly
    adv 1: not easy to believe; "behind you the coastal hills plunge
           to the incredibly blue sea backed by the Turkish
           mountains" [syn: incredibly, improbably,
           implausibly, unbelievably] [ant: believably,
           credibly, plausibly, probably]
    2: exceedingly; extremely; "she plays fabulously well" [syn:
       fabulously, fantastically, incredibly]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "incredibly":
   abnormally, abundantly, acutely, amazingly, amply, astonishingly,
   awesomely, beguilingly, bewilderingly, conspicuously, copiously,
   eminently, emphatically, enigmatically, exceptionally, exquisitely,
   extraordinarily, exuberantly, fabulously, famously, fantastically,
   fascinatingly, generously, glaringly, hardly, hardly ever,
   impressively, incomprehensibly, inconceivably, intensely,
   magically, magnanimously, magnificently, markedly, marvelously,
   miraculously, nobly, notably, outlandishly, particularly,
   peculiarly, phenomenally, pointedly, preeminently, prodigiously,
   profusely, prominently, pronouncedly, puzzlingly, rarely,
   remarkably, richly, seldom, seldom if ever, sensationally,
   signally, singularly, splendidly, strangely, strikingly,
   stupendously, superlatively, surpassingly, surprisingly,
   uncommonly, uncustomarily, unexpectedly, unimaginably, unnaturally,
   unordinarily, unusually, wonderfully, wondrous, wondrously,
   worthily