Green’s Dictionary of Slang

1. (US) a confidence trickster.

1949		N. Algren Man with the Golden Arm 77: This mild-looking, white-haired, stoop-shouldered coneroo.
1951		N. Algren Chicago: City On the Make 74: For [...] stylish coneroos, this is the spot.
1974		(con. 1940s) E. Thompson Tattoo (1977) 24: When his eyes backed and filled and focused on you, they were those of a barroom conneroo.

2. a confidence trick; any act of deception.

1951

C. Stoker Thicker ’n Thieves 244: But Bowron, ever the politician [. . .] proved that he was still the master shover of the old coneroo. He took to the radio and yowled and howled that the gangsters were coming, ta-ra.