Green’s Dictionary of Slang

(Aus.) to speculate (in).

1900		Sydney Sportsman (Surry Hills, NSW) 17 Oct. 1/3: Unless a man has three guineas to spec, the Rats Club will not listen.
1902		W.A. Sun. Times (Perth) 16 Feb. 1/1: Having captivated a well-known married stockbroker she plunged heavily on big mining scrip [and] having specked stock which immediately rose she left the broker to pay the difference.