Thorin acquired the title long before he became King. In T.A. 2799 when he was just fifty-three (a young age for a Dwarf) he marched with a mighty Dwarf-army to the valley they called Azanulbizar, Nanduhirion beneath the East-gate of Moria. There they fought the Battle of Nanduhirion, the last and greatest in the War of the Dwarves and Orcs. In that battle, Thorin's shield was broken, so he cut a bough from an oak-tree with his axe, and used that instead to fend off his enemies' blows, or to club them. It was that oaken branch that gave Thorin his surname, but it did not completely save him from injury - it is recorded that he was wounded in the battle. In memory of the battle, Thorin bore a plain shield of oak wood with no device, and swore to do so until he was hailed king.
All from an ancient oaken-tree
A mighty branch he tore, and lammed those lads so lustily
That some rose up no more
- J.R.R. Tolkien, The Lord of the Rings, Appendix B, "The Third Age"
- J.R.R. Tolkien, The Lord of the Rings, Appendix A, "Durin's Folk"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Peoples of Middle-earth, "IX. The Making of Appendix A": (iv) "Durin's Folk"
- J.R.R. Tolkien, John D. Rateliff (ed.), The History of The Hobbit, Return to Bag-End, "Appendix III. The Dvergatal (The Dwarf Names)"
- Various, "Influence of the Sigurd legend on parts of The Hobbit?", The Lord of the Rings Fanatics Plaza: Forum (accessed 14 March 2013)
- E.M. Smith-Dampier (1934), Sigurd The Dragon-Slayer, A Faroese Ballad Cycle