impassable vs unpassable

impassable

adj
  • Incapable of being passed over, crossed, or negotiated. 

  • Incapable of being overcome or surmounted. 

  • Not usable as legal tender. 

unpassable

adj
  • Not able to be passed. 

  • Not able to be passed; not capable of being beaten at the net by a passing shot. 

  • Unable to pass successfully as the gender one wishes to be seen as. 

How often have the words impassable and unpassable occurred in a corpus of books? (source: Google Ngram Viewer )