“A slave jargon exists among the Nez Percé Indians”
William Wallace Beach (ed.). 1877. The Indian miscellany: Containing papers on the history, antiquities, arts… Albany: J. Munsell.
In Albert S. Gatschet’s article here, “Indian languages of the Pacific states and territories”, page 444 brings this fascinatingly specific observation:
A slave jargon exists among the Nez Percé Indians, which originated through their intercourse with prisoners of war, and contains expressions for eye, horse, man, woman and other most common terms, which are entirely foreign to Sahaptin.
This may be the original published source for the information that there was a separate pidgin used among the Nez Perce tribe. Gatschet was well acquainted with Chinook Jargon, and he mentions it on the preceding page, so he’s not confusing it with this Nez Perce pidgin. He also draws a clear distinction from “Sahaptin”, here meaning the Nez Perce language.
It would be nice to track down whatever original notes provided Gatschet’s information. We might see what the actual words were that he refers to, and get an idea of their source languages.