What does だってばよ Dattebayo mean?

Dattebayo means very little on its own: it is an insisting sentence ending, closest to I'm telling you. It is Naruto Uzumaki's verbal tic, and the English Believe it! was the dub's solution to it rather than a translation of it.

The ending, taken apart

There are no kanji here; the whole thing is grammar. だ (da) is the plain copula, is. ってば (tteba) is a colloquial particle that the Nihon Kokugo Daijiten, on Kotobank, derives from と言えば (to ieba), if one says. At the end of a sentence, the same dictionary says, it presses a point the speaker has already made when the listener has not responded to a request or call; its earliest cited example comes from Futabatei Shimei's novel Ukigumo (1887 to 1889). よ (yo) is the familiar sentence-final particle that, in JMdict's gloss, indicates certainty, emphasis, contempt, request or warning.

Put together: I'm telling you, and I'm telling you again. The emotional color is impatience at not being listened to. JMdict glosses sentence-final tteba as (I told you) already, or come on, and marks it as colloquial.

Is it real Japanese?

The parts are. Tteba is ordinary colloquial speech, a century and more old by the dictionary's evidence. Stacking it after da and before yo, on sentence after sentence, is a character trait. In a 2023 interview on the official Naruto site, the interviewer describes dattebayo, alongside Konohamaru's kore, as speech that is not common even in Japan and belongs to Naruto in particular.

So the register is colloquial and pushy rather than rude: by the dictionary's account, it is the ending of someone who feels unheard.

Believe it

Anime News Network's review of the first English dub DVD (Zac Bertschy, March 24, 2006) describes Naruto's catchphrase Believe It! as used more like punctuation than a standard speech pattern, and adds that the original dattebayo is equally obnoxious. Wikiquote's Naruto page lists Dattebayo! among the recurring quotes with the note that it is Believe it! in the English dub.

Why that phrase? In the same official-site interview, VIZ Media editor Alexis Kirsch says Believe It was the anime's rendering and does not appear in the manga, and that he thinks it came from the dubbing-specific need to find wording that fit the characters' mouth movements.

What the manga did instead

The translator Mari Morimoto, interviewed alongside Kirsch, says she took over the translation from volume 2, noticed on rereading volume 1 that dattebayo had not been translated, and learned from the editor of the time that, after some discussion, the decision had been not to translate it. Where a scene needed it, she rendered it as I tell ya, though she recalls that it hardly ever appeared.

Morimoto gives a related reason for translating other characters' verbal tics, such as Konohamaru's kore, only in scenes that need them: rendered every time, they would no longer fit in the speech balloons.

Why English cannot just keep the ending

The gap is a fact about English grammar. Japanese can mark attitude with small particles at the end of a sentence: tteba and yo add insistence to a line without adding a clause. English has no such slot. Insistence has to be spelled out in words, I'm telling you, you know, believe it, and each of those is a full phrase that competes with the sentence it attaches to.

One reading follows from that: Believe it! is a statement of conviction, a whole sentence of its own, while dattebayo, by the dictionary's account, is closer to irritation at not being heard.