No meaning, at least as far as I know. I wanted to name him Dermot (the Irish name), but for some reason in my head Dermot got mixed up with Dortmund, and by the time I realized the difference I’d come to like Dortmund. I did a google search on the name, and the only thing I could find named Dortmund was a city in Germany and their soccer team. My parents think I must have just made it up. Though I suppose there is always a chance It was a character in a book I’d read somewhere.