The young Naruto was supposed to be understood as an uneducated and dumb ninja.

There is no one word "dattebayo". It is just "datte" with the '-bayo' suffix. Sometimes Naruto would ask, "Nandattebayo?!", which is just "nanda" with the '-ttebayo' suffix. You're best off thinking of it as a plain superlative, like LittleMchale said.

This sort of thing has a root in poetry (adding nonsense but superlative or negative sounding syllables to fit a rhythm). Supposedly, the lower class Yakuza members go around saying -ttebayo all the time now.

Quote Originally Posted by Archangel
I wonder if the -bayo gets annoying for the japanese too. Personally i rarely notice it anymore but then again i focus more on the subtitles then the voice acting
He doesn't do it anymore.