Hum, the word might just have a meaning if you allow yourself to travel in crazy thoughts. If you use it in a context of a society in which each and everyone are considered lost souls, blind in their human stupidity, stumbling around on their own feet, the Underlost would be a kind of a hero, a someone more reasonable, who isn't as lost as all the others. Of course this would be a very specific and a bit crazy usage, but it shows that the word does have a meaning, like, actually, every word does, as long as they have a plausible ethimology.

I know of no meaning for this word in English.

This isn't a word, it's probably made up by someone who thought it sounded nice. :)