A Moose Knuckle can be defined as the big brother of the camel toe. It is a set of pussy lips that are extra long and meaty. Usually found on a skanky pussy that is blown out due to excessive use. Most likely to belong to the town whore. Moose Knuckles generally like to eat yoga pants.
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 "Moose-knuckle"
Jon didn't wear underwear under his skintight Peterpan costume, and everyone could see that he had a moose-knuckle.