A bouncing male package. Usually when a guy wears sweat pants or basketball shorts (any loose/thin material), you can see his package bouncing around.
It got its name from the song Mr. Bojangles:
"I knew a man Bojangles and he danced for you ..."
> Check out the Bojangles on that guy!
> One could also just start singing the song to alert their friends without necessarily pointing out or bringing attention to the fact you're checking out someone's package.