Green’s Dictionary of Slang

1. a militiaman or conscripted soldier, esp. one who was drafted into the WWII militia but never left Australia; also attrib.

1944		L. Glassop We Were the Rats xi: How do you feel about the chocos? And the munition workers?
1946		D. Stivens Courtship of Uncle Henry 26: [She] called out the militia guard. Guard came in at the double with fixed bayonets Nolan got up clenched his fists [...] says ‘Come on, you choco bastards, come on!’.
1948		S.L. Elliott Rusty Bugles II iv: andy: You big galah. [...] keghead: In your great dinger, you rotten crawling chocko.
1959		‘David Forrest’ Last Blue Sea v: Militiamen (Chocolate Soldiers, Chockos) were mostly wartime conscripts. [...] The Chock War, the bitter discord between the A.I.F. and the Militia, began in Liverpool Camp near Sydney in October 1939.

2. a conscientious objector.

1951		Cusack & James Come in Spinner (1960) 24: Hero be blowed. He’s a choco. Caught in the draft. They say her old man put the military on to him but she’s succeeded in keeping him in Melbourne so far.
1968		(con. 1940s) G. Dutton Andy 91: Your country called you and you came. Not a chocko amongst you.