Many words have a number of urban myths associated with them and stories about their origins. chav is one of them. Here in Cheltenham, the story is told that chav was originally coined by Cheltenham Ladies College girls who would rate local lads. They used the word chav to mean Cheltenham average.
The most likely explanation is that chav comes from the Angloromani word chavvy to mean baby, or child. English borrowed the word in the late 19th century and used it to mean baby. The current meaning of chav comes much later in the 1990s and started to be used in the south east of England as a commentary on the dress, style and behavior of some working class youths. Today, it tends to be used as a generalized derogatory term for working class young people.