Phrase used during football (soccer) meaning a player kicked the ball extremely high over the cross-bar from less than 6 yards away. Usually during a very good scoring opportunity.
However this is not used when a player hits it far wide.
"Oh no, Guy3 is doing a jonny"
"Haha, Guy1 just did a jonny"
"Hope Guy4 doesn't do a jonny again"
"Hey man, I bet I am going to do a jonny later"