Is Impassable a Correct Word?

not passable; not allowing passage over, through, along, etc.: Heavy snow made the roads impassable.

Is it unpassable or impassable?

As adjectives the difference between unpassable and impassable. is that unpassable is not able to be passed while impassable is incapable of being passed over, crossed, or negotiated.

What is the base word for impassable?

When you break apart the word impassable, it's pretty easy to figure out what it means. The im- prefix, a variant of in-, may be familiar to you as a way to say “not.” And passable is likely pretty familiar too, referring to something that can be crossed.