Green’s Dictionary of Slang

1. a possibility.

1948

K. Williams Diaries 17 Dec. 36: Says there is a poss. of me getting work at Stratford Rep.

2. (Aus.) a fool, esp. a trickster’s victim [abbr. possum n. (5)].

1944		T. Hartley gloss. in Simes DAUS (1993).
1948		Smith’s Wkly (Sydney) 25 Dec. 6/2: One night the silly poss got some petrol on his sleeve. A bit later he lit a cigarette.
1950		‘No. 35’ Argot in G. Simes DAUS (1993).
1968		J. Alard He who Shoots Last 112: You cheeky old poss.