Green’s Dictionary of Slang

(Aus.) completely, wholly; spec. of an arrest, ‘bang to rights’ .

1904		Sun. Times (Perth) 31 July 4/7: I got me pitcher taken before ’e made me squinny-eyed but me ’usband give ’im the orfice and he copped me fair dinkum.
1906		Sun. Times (Perth) 18 Feb. 4/7: Well, ’e’s gone dead crook. Fair dinkum crook ’e’s gone.
1910		Register (Adelaide) 4 Jan. 10/3: You had me ‘square dinkum’ at the Morphetville races, but you couldn’t find the square dinkum bloke.
1955		S. Abicair [song title] I Love You Fair Dinkum (Dinki Di) I Do .
1983		J. Byrell (con. 1959) Up the Cross 8: ‘I fair dinkum dropped a motzah’.
1984		B. Ellem Doing Time app. C 242: [S]ome people were surprised when I told them I was a druggie, ‘What, sticking needles in your arm and all that fair dinkum!’ .
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 73: ‘I fair dinkum fell for the old coat-tug trick. I must be the worst idiot mug in the world I reckon.’.
2002		R.G. Barrett Mystery Bay Blues 130: Are you fair dinkum going to fight that dopey, big relation of yours.