The Collaborative International Dictionary
Impassable \Im*pass"a*ble\, a. [Cf. Unpassable.]
Incapable of being passed; not admitting a passage; as, an
impassable road, mountain, or gulf.
--Milton. --
Im*pass"a*ble*ness, n. -- Im*pass"a*bly, adv.
Wiktionary
adv. In an impassable manner.
Usage examples of "impassably".
The road was merely a recent clearing through the trees to move the artillery over and was almost impassably muddy.
The day slowly darkened and the rails, as they often did through winter shifterms in Bracebridge, remained impassably blocked with snow.
During her absence, though how or by whom she could not know, the door had been impassably closed to keep her out!
Instead of the fine vistas of his homeland, there seemed only the endless lake, stinking of mud and shadowed by an impassably thick forest.
Then came the first snow, turning the ruins of the curtain wall into an almost impassably slippery barrier, and the familiar necropolis into a strange wilderness of deceptive hummocks, in which monuments were suddenly too large under their coats of new snow, and the trees and bushes crushed to half size by theirs.