Green’s Dictionary of Slang

(Aus.) aware (of).

1885		Bulletin (Sydney) 16 May 11/2: But do not think that once again / Your beauty, Bob, will save you! […] / The fact is, Cupid – bend your ear – / We’re ‘Jack about the siren.’.
1908		E.G. Murphy ‘Andrew Barr of Doodlekine’ in Jarrahland Jingles 58: Brum’s in London on his ‘ace,’ Of Andrew Barr a trifle ‘jack’.
1968		S. Gore Holy Smoke 51: It all begun in the first place with Him gettin’ pretty jack of the way all these booze artists and eeler-spielers and loose women, as they’re called, was lairin’ it up.