Actually, it's 'dattebayo'. No matter how much I search on Google, though, I can never find a solid definition, sorry. I think it's just a habit of his to say it.
Hmm, I never thought about it, but he does say it a lot xD I thought it was just a Japanese phrase. Apparently its a trait he got from his mom (remember that episode when he met his mother?). His mom's saying was translated as "likewise" or something. Dattebayo means "ya know", I guess? For some reason, the dub translated it as "Believe it!" which I thought was just the abridge but its in the dubbed anime...lol.
posted over a year ago
I think it was to make it fit in better with the mouth movements. :3