Green’s Dictionary of Slang

1. unimpeachable.

1898

Hartlepool Mail 21 May 6/5: Oh, I wisht I was a ghost, with a skull for a head / — A really honest-Injun ghost / of someone that is dead!

2. honourable.

1901		Boy’s Own Paper 25 May 530: This camp is honest Injun all through.
1906		Sun (NY) 9 Sept. 3/4: Give me the honest Injun mitt on it.
1922		(con. 1918–19) S.V. Benét Beginning of Wisdom 296: In a month after that we’ll all be honest-Injun Sammies.
1943		Baker ‘Influence of American Sl. on Australia’ in AS XVIII:4 256: honest injun to you is our dinkum.