Green’s Dictionary of Slang

Jack Lang n.

also old Jack Lang
[rhy. sl.; ult. proper name Aus. politician John Thomas ‘Jack’ Lang (I876-1975)]

(Aus.) slang.

[Aus] ‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/3: jack lang: Slang.
[Aus]R. Aven-Bray Ridgey-Didge Oz Jack Lang 32: Jack Lang Slang.
[Aus]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.
[Aus]Pete’s Aussie Sl. Home Page [Internet] Old Jack Lang: rhyming slang.