Green’s Dictionary of Slang

(US) genuine, sincere, proper; also as an excl. of affirmation.

1912		C. Mathewson Pitching in a Pinch 233: A really true, on-the-level, honest-to-Jiminy jinx can do all sorts of mean things to a professional ball-player.
1933		F. Way Log of the Betsy Ann 105: Any common, ordinary mate can know his men on deck [...] but it takes an honest-to-john mate to walk up to a cow.
1948		O. Weston Mother Lode 72: Old-timers insist [this cabin] was the ‘honest-to-John’ hideaway used by the famous California bad man, Joaquín Murieta [DA].
1958		(con. 1945) F. Davis Spearhead 26: You take these people out to meet some of the honest-to-John soldiers we have around here.
1960		G. Swarthout Where the Boys Are 4: All that Atlantic honest-to-John Ocean and genuine sailing ships on it.
1968		R. Moody Horse of a Different Color (1994) 204: Honest to John, I didn’t peep a word to a soul.
2003		M. Hart Rooster in the Sun 157: Honest to John, I dunno what to think.