buttocks, hips, butt, rear
臀 centers on the fleshy lower back part of the body: the buttocks, hips, or rear.
臀 combines 殿 (which may contribute a phonetic element) with 月 (flesh/body part), indicating a part of the body. The exact historical development of the phonetic role is uncertain.
The top part 殿 looks like a grand building, and the bottom 月 is flesh. Picture the fleshy foundation of a seated person—the buttocks—as the 'palace' of the lower body.
For デン, imagine a den—a cozy room where you sit and rest on your rear: den -> デン, and your buttocks settle into the soft den.