Left clothing radical, the side-compressed form of 衣. Indexes 裆 in the garment family (衫 shirt, 裤 trousers, 袖 sleeve) and locks the word firmly to fabric - specifically the cloth join at the crotch of pants, the seat-seam where the two legs meet.
Right 当 supplies the sound dāng directly with no shift - a tidy phono-semantic where the right side reads exactly the same as the whole character. The traditional form 襠 used 當 here; the 1956 reform shrank both 當 to 当 and 襠 to 裆 in one stroke.
