Green’s Dictionary of Slang

(Aus.) a steady boy-or girlfriend.

1914		Sport (Adelaide) 31 Jan. 10/2: Maggie Q. glad to see you have caught Rangie H. for a square dinkum .
1914		Sport (Adelaide) 7 Feb. 7/1: They Say [...] That Gipsy J. and Cockie S. are going strong with their square dinkums .
1914		Sport (Adelaide) 7 Feb. 7/3: Ru. T. has got a ‘square dink’ in Moonta [...] What have you done with the Wallaroo girl, Trippy?