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"

When a man has a camel toe because he's wearing something really tight.

The guy version of cameltoe.

when an obese lady has camel-toe

Camel-toe's fatter, uglier, sister.

a camel toe on a woman of native North American ancestory.

when a women/man pulls up their pants to high they get a "camel toe" in slang a moose knuckle