Whether it’s a road, a barrier, a river, or an abyss, if you can’t travel through or over it, you can describe it as 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. When you combine those parts you get something that you can’t navigate. Impassable is usually used in a literal sense, referring to things that you can’t travel through — like side streets after a major snow storm.
