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.