Green’s Dictionary of Slang

1. a thief, a cheat, a confidence trickster.

1674		Jackson’s Recantation in C. Hindley Old Bk Collector’s Misc. 21: I was generally known as an expert Nicker.
1674		C. Cotton Compleat Gamester Preface: If out, he raps out Oaths I dare not tell, Hot, piping out, and newly come from Hell, Old Nick o’re-hearing, by a Palming-trick Secures the Gamester; thus the Nickers nickt.
1851		Bell’s Life in Sydney 25 Oct. 1/3: [headline] The Nicker Nicked.
1877		J. Greenwood Dick Temple III 11: Look here, Bobby Nicker, [...] you say that you have been in the coop as many times as I have.
1908		B. Fisher A. Mutt in Blackbeard Compilation (1977) 56: A. Mutt the daring phone nicker.
1959		G. Slatter Gun in My Hand 10: Hunka who always carried a B.S.A. spanner, the nicker’s tool is best.
1978		A. Burgess 1985 (1980) 147: Alwyn [...] looked sympathetic, a sly nicker himself probably.

2. (Aus.) a child.

1968

R.D. Magoffin We Bushies 88: I was just a nicker when these lanterns used to flicker.