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