The nickname given to any member of a sports team who neglects to wear fitting boxers or compression shorts and proceeds to participaate in jumping jacks. Their neglecence results in a the very noticable flopping motion of their package as they continue jumping.
You: yo man aint u trippin about forgetting ur compression shorts for practice
Friend: na its aight dude I'll just wear these loose boxers
You: But we doin jumpin jacks right now, dude ur dick is floppin everywhere. Ur a wacker jacks forreal