Green’s Dictionary of Slang

(Aus.) honestly, genuinely; thus dinkum? really? is that so?

1917		W. Muir Observations of Orderly 222: I could see he knew I’d clicked a packet, square dinkum, this trip (‘Square dinkum’ or ‘dinkum’ is an Antipodean verbal flourish, which broadly approximates to the American ‘Sure enough’ or the English ‘Not ’arf’).
1919		C.H. Thorp Handful of Ausseys 163: Just a quid ter carry me on sarge. I’m broke to the wide, dinkum.
1919		C.H. Thorp Handful of Ausseys 272: Yer ain’t askin’ that question dinkum, are yer?
1928		V. Palmer Passage 22: It’s someone you know, dinkum.
1938		X. Herbert Capricornia (1939) 310: Tell me, Dad, dinkum — is it true? [Ibid.] 522: ‘I was only kiddin’ her.’ ‘Go on with you!’ ‘Dinkum.’.
1941		D. Stivens ‘A Bum Actor’ in Mann Coast to Coast 14: ‘He wanted one you couldn’t open from inside.’ ‘Dinkum?’ the clerk asked.
1949		R. Park Poor Man’s Orange 190: Dinkum, I hadn’t the faintest that they’d go on like that.
1951		D. Stivens Jimmy Brockett 32: ‘Just a few words. Easy money for a few words, eh?’ ‘Dinkum, mister?’.
1959		G. Slatter Gun in My Hand 210: ‘Oh, you’re just saying that.’ [...] ‘No, I’m not. Dinkum.’.
1960		A. Seymour One Day of the Year (1977) II ii: I ’eard the Last Post. Dinkum, I though they was comin’ for me.
1983		J. Byrell (con. 1959) Up the Cross 10: ‘Dinkum, son. You just wouldn’t credit it’.