User blog comment:Coby413/D for Determination/@comment-5640399-20121105011216

It's not a middle name, though, it's part of their surname, Portagas D. Rouge was going to call her son Gol D. Ace, not Gol Ace. It's the same with Luffy and his family. It's a heritage, most likely linked to the Void Century and the founding of the World Government, in my opinion. If it stands for anything, I reckon it'll be the name of the country that opposed the World Government in the Void Century, though I've forgotten most of the info about said country...