In*nav"i*ga*bly

from The Collaborative International Dictionary of English v.0.48

Innavigable \In*nav"i*ga*ble\, a. [L. innavigabilis : cf. F.
   innavigable. See {In-} not, and {Navigable}.]
   Incapable of being navigated; impassable by ships or vessels.
   --Drygen. -- {In*nav"i*ga*bly}, adv.
   [1913 Webster]