The definition of the adjective impassable is impossible to pass, cross, or overcome. Impassible originally had a different definition—namely, not subject to suffering, pain, or harm. The words are homophones, but they have separate origins. Impassable is related to the verb pass, which has origins in the Latin passus, meaning step. Impassible is the antonym of passible, which comes from the Latin passus, a past participle of to suffer.
Though impassable and impassible are separate in both meaning and origin, impassible is in danger of either disappearing from the language or becoming a mere variant of impassable. In fact, in our research, we were unable to find any instances of impassable used in its original sense. It is always used to mean impassable—for example:
Overnight snowfall made sections of the route impassible … [Calgary Herald]
Deputies said that some secondary roads are impassible. [Chicago Sun-Times]
Heavy truck traffic from a construction project is making the road impassible for passenger vehicles. [Grand Forks Herald]
We probably have to accept that the original sense of impassible is mostly dead. The same is true of the less common passible. As careful writers, though, we at least can still use these words correctly.
