dag, n.¹ — Green’s Dictionary of Slang

dag n.¹

[dial. dag, a feat of daring]

(Aus.) a feat of skill; thus be a dag at, to be an expert at.

1937–84		Partridge DSUE (1984) 287/1: from mid-1890s.
1938		Press (Canterbury) 2 Apr. 18: ‘To be a dag at,’ ‘to put across a beaut,’ ‘to jerry to,’ ‘ducks’ breakfast,’ ‘to float up to,’ ‘to blow up to,’ ‘to sleep in the Star Hotel’ need no explanation.