In my case, it is referred to as a loaf of bread with veins.
Smugglin’ plums
Now I’d like to Google up a real moose’s knuckle so I can see if it looks like the human counterpart, but I ain’t gonna do it at work, no sir.
Depending on the guy I would call it either gross or an invitation. YMMV.
The Pirate.
(may not be safe for work.)
There’s nothing wrong with his ass. It’s just that he doesn’t carry much fat on his ass, and he’s clenching his glutes, which makes the muscles stand out more. The “third butt cheek” is the small amout of fat that’s on the inside bottom of his buttocks.
Another for plum smugglin’. I picked that up from Mr J’s comments about a coworker who would show up in snug, yellow dress pants.
Hmmm…see, I wanna like “mooseknuckle”, but according to UrbanDictionary.com, it generally refers to a cameltoe (usually large). True, one of the definitions refers to the male utilities, but most of them refer to women’s bits. 
Dear God, this may be the first time I have ever actually laughed at anything on they internet.
Another vote for “lunchbox”.
You and me both. That little lower middle protuberance from his butt crack made me giggle like a office muzzled loon.
My hubby calls it “scrotecrease”.
Spy magazine had a semi-regular feature consisting of pix of [usually B- and C-level] male celebrities called “Man At His Best”.
Sometimes you could tell what religion the guy was. (Well, almost.)
Camel hump is another referance to it, though “unit” has been used as well.
Grape huggers.
Camel tail.