1812 (from 1683 as Ingin), a spelling representing the early American English colloquial pronunciation of Indian (q.v.). Honest Injun as an asseveration of truthfuless is first recorded 1868, from the notion of assurance extracted from Indians of their lack of duplicity in a particular situation.
"Honest Injun?" inquired Mr. Wilder, using a Western phrase equivalent to demanding of the narrator of a story whether he is strictly adhering to the truth. ["The Genial Showman," London, 1870]
The noun phrase honest Indian itself is attested from 1676 in Massachusetts.