Where did "dattebayo" come from? In Naruto, at least in the early show, he uses the catchphrase "dattebayo" in the Japanese. This question answered on a canon-level where it came from (his mother), but I was wondering where (if anywhere) it came from in terms of the language. I know that there's no direct translation, but is it a sort of portmanteau or version of somebody's name or of some phrase in Japanese?
Dattebayo (だってばよ) is not really proper Japanese, but you can sort of see how it would come about linguistically as a sentence-ending phrase. It's very impolite, somewhat childish, and basically just combines different ways of putting emphasis on the statement in a not terribly meaningful way.

First, the da (だ) is a standard way to end a sentence (copula) in Japanese. Depending on the construction of the sentence, some sentences will end with da while others end with a dictionary form verb. It should be emphasized that using this would not be considered polite. The polite version of da is desu (です). Wikipedia has some more information on copula verbs and verb conjugations in Japanese.

Next, tteba (ってば) is sort of a set expression/phrase which adds emphasis. This is a shortened (and hence less polite) version of tteieba (っていえば). It can be attached to a noun, in which case it would be translated roughly as "speaking of" in many contexts. In other contexts it could be interpreted (somewhat rudely) as "I'm talking to you" or "listen to me". You might use it with a person's name to attract their attention in this way. It's roughly equivalent to ttara (ったら)　but this latter one is more commonly used by females. Wikipedia calls it a "strong emphasis marker" which is probably about as close as you can get to an accurate English description.

Naruto adds it to the da above. Datteba is something you might hear occasionally in other contexts. To me, this more often sounds frustrated than emphatic, but it could be used either way.

Finally, the yo (よ) is a sentence-ending particle. It can be appended to the end of a sentence when the speaker is informing the listener of something or making an assertion. It might be translated in some contexts as "you see" or "you know". It's not necessarily impolite but it could be used in impolite speech. Wikipedia has some other contexts where this could be used.

Putting all these together gives dattebayo. It's meaningless and not entirely grammatical, but it puts a lot of emphasis on what he is saying. One might compare it qualitatively in English to using multiple superlatives, e.g. "I'm the most bestest ninja ever" in terms of how it would come across. It isn't actually equivalent to this, but Japanese speakers hearing it would get a similar impression in terms of the manner of speech.

I'll note that while dattebayo is most definitely invented for this character, it's somewhat equivalent to dazo (だぞ). Dazo is not childish-sounding (compared to dattebayo), and is mostly used by men. However, it is still quite impolite, all things considered, and puts a strong emphasis on a statement. There's also the more forceful daze (だぜ), but dazo has somewhat of a more positive tone.