Malley’s (cow), n. — Green’s Dictionary of Slang

Malley’s (cow) n.

[? the story of one Malley who was supposed to look after a cow. When his boss returned to find Malley but no cow and asked what had happened, he received the reply ‘she’s a goner’]

(Aus.) one who has left, gone away.

1951

E. Hill Territory 445: Malley’s Cow*: A person gone away. [*Back in Monroe folklore one Malley in a mustering camp was told to hold a particular cow. When the boss came back and asked for it, Malley grinned. ‘She’s a goner!’ he said. Hence the proverbial Malley’s Cow].