Green’s Dictionary of Slang

1. (N.Z.) a double portion of a drink, two shots of a spirit in the same glass.

1871

C. Money Knocking About in N.Z. 141: We reached the Wangangaro Redoubt [...] and were served out with a ‘doubler,’ or two lots of grog in one.

2. (UK Und.) a corrupt police officer who not only takes the offered bribe but still arrests one for the crime.

1981

(con. 1900–30) A. Harding in Samuel East End Und. 282: Doubler – A bent policeman who will cop your money and do you at the same time.

3. (Aus., also double) a lift on a bicycle crossbar.

1920		Morn. Bulletin (Rockhampton, Qld) 16 Jan. 4/3: He was getting a ‘double’ on a bicycle, and, while crossing the Fitzroy Bridge, the machine came to grief.
1925		Dly Mail (Brisbane) 15 Feb. 1/2: Early this morning two lads [...] were riding a bicycle double along William-street, when the front fork broke, and they were thrown heavily to the ground.
1933		Queensland Times (Ipswich edn) 28 Nov. 6/4: When Frank and Clifftord Young were riding a ‘doubler’ on a push bicycle yesterday, the bicycle skidded and threw them to the road.
1935		Tweed Dly (Murwillumbah, NSW) 1 Mar. 4/3: Davis [...] was giving a younger lad a ‘double’ on his bicycle [...] when a boot of the latter caught in the machine and both were thrown heavily.
1937		Eve. News (Rockhampton, Qld) 29 July 2/2: Another danger which the club appeals to push-bicycle riders to refrain from is that of ‘giving a doubler’ as the act of carrying a passenger is generally de scribed.
1945		Baker Aus. Lang. 72: Doubler, dink and dinkie are interchangeable variations [on double-bank v.].

4. (UK drugs) a double-sized balloonful of nitrous oxide (aka laughing gas).

2009

R. Milward Ten Storey Love Song 26: He sets them up with doublers, passing Ellen a big juicy watermelon-size [balloon].

5. a double payment, a double share.

2013

T. Black Ringer [ebook] n.p.: Wee Bri was a greedy cunt; twenty sheets a night I started paying him, and there he was last week asking for doublers.