a "jock jack" is simply the term for when you are doing jumping jacks and your jock bounces all around which later results in an unpreventable boner. This occasionally can be a problem if females are around in your intense workout session because floppage of the package can sometimes cause boners from the friction. This is an awkward thing to happen cause your are getting your cardio in and all of the sudden your johnson is erect.
by The one who is anonomous October 22, 2009