With regret, I provide this citation as the context I use the word in, as I note there is more than one definition:

Injun

1812, spelling representing Amer.Eng. colloquial pronunciation of Indian (q.v.). Honest Injun as an asseveration of truthfuless first recorded 1876 (in "Tom Sawyer"), perhaps from the notion of assurance extracted from Indians of their lack of duplicity. The term honest Indian is attested from 1676.

Online Etymology Dictionary, © 2001 Douglas Harper
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)