Green’s Dictionary of Slang

1. anything large or substantial.

1861		C.C. Robinson Dialect of Leeds 386: ‘A plonker’ is an article having extraordinary substance. A piece of woven material unusually thick is ‘a plonker’ .
1885		Pudsey Almanack and Hist. Register Mar. n.p.: Sitha Bill at that young woman’s improver, isn’t it a plonker? [EDD].
1898		B. Kirkby Lakeland Words 114: Noo that’s a plonker.
1905		J. Wright EDD IV 550/1: That turnip’s a plonker .
1918		Aussie (France) 18 Jan. 3/1: [of a shell] Fritz was putting over some big stuff. Every time a plonker landed near them, one of the officers energetically fired his revolver into the air.

2. (also plonk) the penis [note plonk v. (3)].

c.1920		implied in pull one’s plonker
1970		B. Aldiss Hand-Reared Boy 54: Can I get it out? [...] Your thing. Your little plonk.
1988		J. McDonald Dict. of Obscenity etc.
2000		N. Barlay Crumple Zone 226: He’ll believe whatever’s in his hand that’s not his plonker.
2012		(con. 1980s) I. Welsh Skagboys 215: I swivel my hips, watching my big placky plonker move fae side tae side.

3. (also plonk) a general term of abuse [widely popularized by the 1980s BBC TV series Only Fools and Horses].

[1959		A. Zugsmith Beat Generation 141: Listen, plink-plonkers and hipchicks].
1966		J. Gaskell All Neat in Black Stockings 72: If she’d been my daughter in fact I’d never have let her go out with an obvious plonker like myself.
1970		Sun. Truth (Brisbane) 13 Sept. 36/2: Do you know what a plonker is?—It’s a chap who shares his ladyfriends with his mate [OED].
1981		J. Sullivan ‘Cash and Curry’ Only Fools and Horses [TV script] What a plonker!
1992		in R. Graef Living Dangerously 223: I know the guy. He’s a plonk.
1993		C. Fowler Darkest Day (1998) 334: He was a plonker of the first order.
2006		D. Mitchell Black Swan Green 171: You prat [...] you ponce you pillock you plonker.
2020		Times 3 Oct. 🌐 [headline] Only a plonker would call time on sozzled bonking.

4. (Aus.) a bettor, who ‘plonks down’ their money.

1996

J. Byrell Lairs, Urgers & Coat-Tuggers 306: ‘We thought maybe just the old faithful plonkers we’d been used to would have a go’.