Extract from OED Online

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 Sunday Truth (Brisbane) 13 Sept. 36/2 Do you know what a plonker is?It's a chap who shares his ladyfriends with his mate. 1987 Match Fishing Feb.-Mar. 52/1 To say that I was a plonker at the game was an understatement, but I wanted to learn. 1988 Smash Hits 19 Oct. 9/3, I look at a dress and think because it's fashionable it'll look good and then I go out with it on and realise what a plonker I look. 2000 H. SIMPSON Hey Yeah Right get a Life (2001) 69 You're going to look a right plonker when the end of the world doesn't come, aren't you?