Not so much an etymology, but how it was used in pop culture:

Our local paper used to publish a cartoon and poem every fall. The piece was called Injun Summer, and it was printed every October from 1907-1992.

It's very much a relic of its era, which is to say "it was weird; really fucking weird." The image is lovely. The text is an old man telling a young boy a totally made up story. It's folksy, wistful and nostalgic. It talks about the past and how native spirits (literally ghosts) return to the land each fall. It's also written in the vernacular of what an old man in 1907 might sound like.

Personally, I don't think the complaints about racism were what caused them to stop printing it. I think it was the weirdness that just didn't appeal to anyone under the age of 50 (in 1992!).

The fist link shows the image with text. The second shows how it would have looked in print.

http://www.sewwug.org/images/injun_summer_2.pdf

https://drloihjournal.blogspot.com/2017/10/the-history-of-john-t-mccutcheons-1907.html