I’ve heard it infrequently all my life, growing up in Idaho and Oregon. I think it’s one of those rare words of Northwestern origin that has made its way into the lexicon - the Northwest was one of the last parts of the continental U.S. to be settled and consequently most of our dialect is imported from somewhere else.
In ordinary use as an adjective it usually means “tough” or “durable”, as describing someone you don’t want to pick a fight with. The related "skookumchuck" is common in place names…I know of a Skookumchuck Creek in Northern Idaho and Wikipedia lists several others. The Skookum name was also given by Northwest lumbermen to a class of locomotive that was designed for use on logging railroads.