Starting off with expensive things that other poor kids couldn't afford, this was a snobbish insult used by richer kids who had the flashest new gear and one of the poorer kids wanted to touch it as they could never afford somethign so extravagent. This was in fear of the poorer kids leaving germs or nits] on the item leaving the richer kid fucked later on. This would put them in their place and the poor kids would know their place in society.
However, things started to get a bit out of hand circa 1997. The diss spread wildly across Britain's schools and soonn became used in day to day conversations heard in the playground; the cheaper the tackier the item the better the insult. It was a witty way of saaying "you're so poor that..." but without having to use the brainpower to think of an item and and it would leave people hurt inside.
Soon enough people found a way of responding to this comment which was to get your wallet out and show you have enough money to indeed afford the item, thus making the person who used it look silly and make their point completely void. Soon after this was discovered people stopped using it, and thus sending a classic diss to the history books along with "your mum gives head for bread" and other such insults.
Person 2 "Oi, don't touch what you can't afford!"
Person 1 "Damn...you didn't have to go there."