Green’s Dictionary of Slang

(Aus.) slang.

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/3: jack lang: Slang.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 32: Jack Lang Slang.
1999		G. Seal Lingo 90: jack lang means slang, though may also be applied specifically to rhyming slang, as in to talk the old jack lang Jack Lang was a famous or infamous, depending upon your political views, Labor premier of New South Wales during the depression years of the 1930s.
2002		Pete’s Aussie Sl. Home Page 🌐 Old Jack Lang: rhyming slang.