Wordnet 3.0
ADVERB (1)
1. 
 to a degree impossible of achievement; 
- Example: "long thought to be an impossibly difficult operation"- Example: "impossibly far from sources of supply"
The Collaborative International Dictionary of English v.0.48:
Impossibly \Im*pos"si*bly\, adv.
   Not possibly. --Sir. T. North.
   [1913 Webster]
WordNet (r) 3.0 (2006):
impossibly
    adv 1: to a degree impossible of achievement; "long thought to
           be an impossibly difficult operation"; "impossibly far
           from sources of supply" [ant: possibly]