Saying popularised in Glaswegianfootball, in particular by Old Firm fans. This saying refers to the ex-manager of Celtic Football Club Jock Stein. In the early 1970s, young boys were abused by a youth coach at the club called James Torbett. Stein aka Big Jock is reputeded to have known about this and not reported the matter to the authorities, and also been part of a cover up within Celtic Football Club.
Hence the saying "Big Jock Knew".
Celtic fan : "Ya F***ing hun basturt!"
Rangers fan : "Away ye go ya wee dick. Big Jock Knew!"