Slapping a random person (they can be on the bus, walking down the street, whatever) whilst recording the act with a video camera phone. It's a very unfortunate London phenomenon. Attackers are usually yoots (ghetto versions of chavs) thinking they are rudeboys. Victims are usually boys around the same age as the attackers, but it's not unknown for victims to be older, or even women. Sometimes the tables are turned, as there are videos going around where the attacker is happy slapped by the victim. More power to the victims, I say. The act is totally random, so be on the lookout for a group of pussified yoots pointing their camera phone (that their mum bought) at you.
If you get happy slapped, grab the camera phone and smash it on the ground.