Green’s Dictionary of Slang

cock-sparrow adj.

[rhy. sl. cock-sparrow (pron. ‘sparra’) = yarra adj.]

(Aus.) mad, insane.

[UK]Partridge DSUE (8th edn) 233/2: later C.20.
[Aus]Tupper & Wortley Aus. Prison Sl. Gloss. [Internet] Cock sparrer. Mad. Rhyming slang for ‘Yarra’. Yarra Bend (Vic) was the site of a mental institution.