Forn

This is the name the Dwarves used for Tom Bombadil.  However, it should not be taken as a Khuzdul word.  In fact, it is an Old Norse word meaning "ancient".  Because Tolkien used names found in Scandinavian myths to represent the outer names of Dwarves, I think we can safely say that this name follows the same vein.  It is probably an old Mannish name or word, or represents such.

The Treason of Isengard, pg 125, 138