Green’s Dictionary of Slang

(Aus. Und.) a fool, a dupe.

1945		S. Baker Aus. Lang. 142: I append a vocabulary supplied to me by an ex-con man. […] Here is his glossary with the Australianisms italicized and some notes on the non-Australian elements: alec, dill, gay, sim, lolly, bunny, and mug.
1999		G. Seal Lingo 50: Terms for a mug [...] include alec, dill (long since promoted to the greater Lingo as a term for a fool); sim; lolly and the accurate bunny.