If you don't know then it doesn't matter.

[ he does a back flip using the force of the shove, distancing himself from the man and huffing irritably. ]

So? A person can be obnoxious and rude and an idiot but still have the strongest heart and will. Luffy's going to become the Pirate King and change the world. I wouldn't call anyone 'captain' unless they were one of my brothers!