Is 'yeah' a proper word?

I suspected that it was something like "géasí" (= "gǽasí(e)") originally, but the thing is that I just generally do not have access to the best of sources on these kinds of matters (as I do not have much access to the kind of stuff that primarily shows up in university libraries or which you have to pay for if you do not have some sort of institution such as a university paying for it for you). Hence I generally have to stick with the stuff one finds in the likes of Etymonline myself.

Yeah is informal. When in a formal setting say "yes..."