Green’s Dictionary of Slang

1. (orig. US) on my honour; also as interrog. meaning ‘really?’.

1851		L. Clappe in Shirley Letters (1949) 52: Instead of simply asking you if it is true, he will invariably nod his head interrogatively, and almost pathetically address you with the solemn adjuration, ‘Honest Indian?’.
1876		‘Mark Twain’ Tom Sawyer 31: Ben, I’d like to, honest injun; but Aunt Polly —.
1881		Manchester Courier 20 Aug. 10/2: ‘That’s honest injun?’ ‘True as I live’.
1884		(con. c.1840) ‘Mark Twain’ Huckleberry Finn 69: I said I wouldn’t, and I’ll stick to it. Honest injun.
1896		P.L. Dunbar ‘A Confidence’ in Lyrics of Lowly Life 174: Honest Injun! this is true, / Ever’ word I’m tellin’ you!
1901		D. Cotsford Society Snapshots 113: Sir Startin Price (à la Didymus) Honest Injun? Lady de Handicap (in his ear) Diddleums! Diddleums!
1902		S.E. White Blazed Trail 73: ‘Our compact holds now, honest Injun; doesn’t it?’ asked the boy anxiously. ‘Honest Injun,’ laughed Thorpe.
1910		Magnet 27 Aug. 7: Then I’ll settle up, honest Injun!
1917		Van Loan ‘The Last Chance’ in Old Man Curry 121: Honest Injun? [...] Is he going – really?
1922		Joyce Ulysses 288: So he took a bundle of wisps of letters and envelopes out of his pocket. – Are you codding? says I. – Honest injun, says Alf. Read them.
1925		Dos Passos Manhattan Transfer 19: Honest injun I’ve got a two-dollar raise.
1926		Hull Dly Mail 14 June 9/1: I went later to see if i could get him out on bail, but ‘honest injun’ poor Mike was — past it.
1938		(con. 1900s) J.B. Booth Sporting Times 177: Honest injun — white man to white man — what’s your form at golf?
1940		C. Rawson Headless Lady (1987) 17: I’ll give some attention to those proofs. Honest Injun, cross my heart.
1941		N.Y. Herald Trib. 29 June 9/2: ‘Fair dinkum’ is the traditional Australian equivalent to bona fide or ‘honest injun’.
1950		A. Buckeridge Jennings Goes To School 217: ‘Honest injun?’ ‘Yes.’.
1966		L.L. King One-Eyed Man 131: ‘Honest?’ the Lieutenant Governor gushed. ‘Honest Injun?’ ‘Honest-goddamn-Injun.’.
1979		Maledicta III:2 161: Injun, honest n phr [DA 1892] Equivalent of ‘on my honor,’ originally sarcastic usage; from alleged dishonesty of Indians.
2015		S. King Finders Keepers (2016) 237: ‘I won’t Morrie, I won’t.’ [...] ‘Honest Injun!’.

2. an affirmative phr. meaning really, definitely.

1902		C.L. Cullen More Ex-Tank Tales 221: You ought to cut it out, honest Injun, you had.
1974		J. Lahr Hot to Trot 110: Honest injun, Fred. Page two. Volume three.