cap one’s lucky, v. — Green’s Dictionary of Slang

cap one’s lucky v.

[SE cap, to protect + lucky; i.e. to take advantage of one’s opportunity to run away]

(Aus.) to run off, to leave at speed.

1881		Trumble Sl. Dict. (1890) 10: Cap-your-lucky. Run away.
1895		Crowe Aus. Sl. Dict. [as 1882].
1941		Baker Popular Dict. Aus. Sl.