Green’s Dictionary of Slang

1. one who lives alone in the outback.

1941		Baker Popular Dict. Aus. Sl. 56: Poon, a lonely, somewhat crazy dweller in the Outer Beyond. [...] A simpleton or fool.
1945		Baker Aus. Lang. 97: Another outback term for a person who lives alone is poon.
1988		McGill Dict. of Kiwi Sl. 86/2: poon loner [...] possibly Aboriginal, possible link English public school word meaning to be unsteady, from Latin ‘ponere’, to place.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

2. a simpleton, a fool, a useless person.

1920		Advocate (Burnie, Tas.) 5 June 7/2: The silly poon’s gone dopey about that clue he’s pirating.
1940		M. Marples Public School Sl. 60: Another considerable group of words in recent use has a definite trans-atlantic flavour, as, for example: boob [...] goof [...] mutt [...] poon.
1988		McGill Dict. of Kiwi Sl. 86/2: poon [...] idiot.
1999		G. Seal Lingo 127: In between [the extremes of insult] lies an enormous and subtly graded range of possibilities that include the following: [...] poon; prat; prawn; prick.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].