I think it speaks of one who refuses to accept the chastening of the Lord, who,though warned and punished, continuies in disobedience, rebellion, unbelief, and unrepentance turned over to a rebrobate mind.
I don't think people can backslide and get saved again. I think we would classify someone as backslid, when many times they are being disobedient and subjecting themselves to the chaseting of God (such as David w/ Basheeba). BUT if one truly backslides and does get to the point spoken of in
Hebrews 10:26, I think that is where the warning in
Hebrews 6 comes in so strongly "it is impossible to rnew them to repentance..."
Basically in my view, I don't believe your once saved, always saved, nor do I believe salvation is so fickle that you (can possibly) posess it and lose it multiple times in a lifetime. I think once you have been saved, it is difficult to be lost, but still possible. But IF someone did lose their salvation, it would be impossible to "get saved again". I think the Bible backs up my point, though I admit this hasn't been one of the topics I've spent much time on.