Source : Webster's Revised Unabridged Dictionary (1913)

Impossibly \Im*pos"si*bly\, adv.
   Not possibly. --Sir. T. North.

Source : WordNet®

impossibly
     adv 1: in a manner or to a degree not possible of conceiving; "he
            is impossibly idealistic" [ant: {possibly}]
     2: to a degree impossible of achievement; "long thought to be
        an impossibly difficult operation"; "impossibly far from
        sources of supply" [ant: {possibly}]