Green’s Dictionary of Slang

1. an old horse.

1966

G. & K. Swarthout Whichaway (1967) 7: You’re an ugly, slobbery, old mill rider’s kack.

2. (Aus.) luck [? i.e. link to luck n.].

1965

J. O’Grady Aussie Eng. (1966) 52: Kack. Luck—but never good.