Simply put, nobody knows. There are several theories though, that it comes from the guaraní, from the tehuelche/mapuche, or that it is some sort of deformation of an immigrants language (italians most likely).
The key is, in my opinion, finding out at least the oldest (local) written registry of the word available. If the use of the word goes back to colonial times then probably the guaraní theory is correct (che is also widely used in Paraguay and parts of southern Brasil). On the other hand, if the record goes back to the late XIX-early XX century, then the immigrant theory is more accurate.
In other words, i dont know.