Green’s Dictionary of Slang

(Aus.) a small stone suitable for a missile.

1928		Queenslander (Brisbane) 27 Sept. 6/1: The ‘doctor’ would have in his mouth all the time a small stone shaped to fit into the roof of the mouth, and known as a ‘Coon-die’ stone. After sucking for a while, he would produce this stone from his mouth, this being regarded as the cause of the pain, which had been sucked away in liquid form by the ‘doctor,’ and hardened into a stone in his mouth! The stones were regarded as lucky tokens.
1941		Baker Popular Dict. Aus. Sl.
1968		S. Gore Holy Smoke 13: All I needeth is me shanghai here, and a few of these big coonies that’s layin’ around, and I’ll get stuck into it.
1978		W. Lowenstein Weevils in Flour 378: One of our blokes threw a great big connie at a policeman who stuck his head out.
1981		D. Stuart I Think I’ll Live 316: He hoys me with his right hand, then he heaves this coondie over the fence [...] I went inside and takes the paper off this rock.