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1.
(noun, plural, Brit.) Personalised registration plates on a motor vehicle that suggest to everyone but the owner that the vehicle's occupants are twats, chumps, or under-endowed fuckwits with all the charm and sex appeal of a road accident.
Examples of 'twat plates':

A55 HO7E
N0 P3N15
5AGGY VA91NA
by Just_Tom September 11, 2010