Unprovoked attacks on individuals made in order to record the event, and especially the victim's shock and surprise, on video phones.
Happy slapping - otherwise known as bitch slapping, began as a youth craze in the UK in late 2004. Children or passers by are slapped or otherwise mugged by one or more of a gang while others record the event on video and then distribute it by phone or Internet.
There's nothing really happy about grevious bodily harm.
Initially the attacks were, as the phrase would have us believe, fairly minor pranks. Slapping is street slang for any form of violence, and as the craze spread the attacks became more vicious - often serious assaults known in legal circles as grievous bodily harm. The attacks, which were often spontaneous and random in school playgrounds and on public transport, became more organized, often undertaken in areas were no CCTV cameras or witnesses could record the events. One theory as to the motivation is that members of street gangs are frequently in danger of violence and to be seen to be dangerous provides a degree of safety. Attacking other street gang members will cause them to retaliate whereas attacking random members of the public is less risky. It is important in those circumstances that others witness one's violence otherwise there's no benefit to one's reputation.
The mentality of the adherents to this craze is clear from one posting in 2005 in an Internet forum used by such: "If you feel bored wen ur about an u got a video phone den bitch slap sum norman, innit."
'Norman' is a slang term for a straight, conventional person, which for the street gangs that indulge in this includes pretty well everyone else. It came into the language during the time of the Thatcher government that included Normans Tebbit and Lamont.
Happy slapping itself was no doubt coined as the antithesis of 'happy clappy', a state of mind that would be despised by the street gang muggers who prefer 'slapping'.