Green’s Dictionary of Slang

a youth who steals cars to use in a crime or for amusement; developed into a non-specific term of abuse.

1997		B. Morrison As If (1998) 96: He was a TWOCer, a taker-away of cars.
1999		Guardian G2 25 Mar. 6 When I get older I’ll be a twocker.
2001		OnLine Dict. of Playground Sl. 🌐 twocker, twok, twoc adj. Orig. meant car thief, often ‘ram raider ’ or ‘joyrider’ [...] but eventually just meant anyone a bit dodgy, as in ‘he’s a bit of a twocker, but he’s alright.’.
2003		Observer Crime 27 Apr. 28: A Twocer is someone who steals vehicles etc.
2005		B. Hare Urban Grimshaw 3: There’s Sparky, the twocker, and there’s Urban, the thief.