Simply put it is the male equivalent of a woman's camel toe. Usually caused
by the seam of tight
but thin leg wear such as spandex, the testicles
and penis form two bulges on opposite sides of the seem that vaguely
resembles the hoof, or "knuckle" of a moose. hence
Jon didn't wear underwear under his skintight Peterpan costume, and everyone could
see that he had a moose-knuckle.