ISJ stands for Ian St John, the legendary ex-liverpool football player, and the phrase is used to tell someone to "fuck off". The history behind an ISJ is this: Ian was not the politest person when faced with an autograph request. In his broad accent, and with a hand gesture pointing over his shoulder with his thumb, he used to tell them to "Fuck off and leave me alone laddie". From here on, should anyone ever need to be told to "Fuck off" at work but without offence being caused to the rest of the office then the 3 letter acronym is used and they are told (often complete with hand gesture) to ISJ
After point over your shoulder with your right thumb say in a loud voice "Oi laddie, why don't you just ISJ"
by Jimmy Greaves June 20, 2004