The Rice University Neologisms Database

[Slang] Contraction of “at you,” used in fast speech or as a shorthand form in writing Literally means “at you,” and taken from “at you,” which in fast spoken English gets contracted so that it sounds like one word, “ach-ya.” Acha represents the written version of this word. It is not so much a part of speech as it is a whole prepositional phrase. The writer probably wanted a clever way of ending the e-mail, so she used acha rather than simply spelling out at you. It was also a more informal closing than the standard “sincerely” or “regards,” etc, and well suited to the overall tone of the message. Additionally, acha, because it is spelled phonetically and thus represents spoken English, makes the conversation seem more like it is taking place in real time, because this is how you would hear it in fast English were you actually talking to the other person.