Injun is a slur used to describe Native americans in a derogatory way. Pioneer-related term dates back to the 19th century during the Western Expansion. Probably comes from country people who could not pronounce the word Indian correctly.
The word Injun is a racial slur that has been used historically to degrade and dehumanize native americans. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to native americans. If you are unsure about whether or not it is appropriate to use the word Injun, it is best to err on the side of caution and avoid using it in any situation.