A chav is a young lower-class person typified by brash and loutish behavior and the wearing of (real or imitation) designer clothes. Some people believe chav is a derogatory term for a working class person but this is wrong as a chav does not work. Instead a chav lives off social benefits and breeds with other chavs thus creating a social cluster f*ck and are the sort of TV cannon fodder for reality shows where tension is created by the results of a paternity test. The US equivalent is perhaps "trailer trash".
What's a chav?