Green’s Dictionary of Slang

(Aus./N.Z.) truly, certainly.

1915		G.F. Moberly Experiences ‘Dinki-Di’ R.R.C. Nurse (1933) 12: Dinki di, boy, how sadly I miss you [AND].
1918		Aussie (France) VII Sept. 11/2: I ask you now, as man to man, no kidding, but ri’buck square dinky die, are they the kind of places that we’d pick if they left it to you and I? Not on your life!
1935		C. Drew ‘The Bone-Head’ in Bulletin 6 May 28/1: If everyone had their rights I’d be a star patient in Giggleville [i.e. a psychiatric institution].
1941		A. Gurney Bluey & Curley 23 May [synd. cartoon strip] Speak dinky di, fair dinkum, blanky, bonzer Australian.
1950		‘Neville Shute’ Town Like Alice 82: You say you’re English, dinky-die? All the way from England?
1968		S. Gore Holy Smoke 47: Dinky die – was it three, or wasn’t it?