臀

Bottom meat-flesh radical, the body-part form of 肉. Despite the visual resemblance to 月 (moon), this is a different glyph used exclusively for anatomy: 胸 chest, 腿 leg, 脏 organ, 背 back. It indexes 臀 firmly in the body-parts family and tells the reader to read everything above it as flesh, not as architecture.

Top 殿 supplies the sound diàn → tún with the regular d/t alternation seen across this phonetic series. 殿 also carries a faint semantic kick: in palace architecture it means the rearmost building, the one set behind all the others - exactly the part of the body 臀 names. The buttocks are anatomically the rear hall of the human frame.