coccyx, backbone, hips
骶 centers on the lower end of the spine: the coccyx, the backbone's base, and the surrounding hip area.
骶 combines 骨 (bone) with 氐, which likely contributes the sound and relates to the base or bottom, fitting the coccyx as the base of the spine.
The bone radical 骨 on the left points to a body part, and the right side 氐 looks like a person sitting low. Picture the lowest bone of the spine, the coccyx, right where the hips meet.
For テイ, imagine a tea cup resting on the very base of the spine: tea -> テイ, and the coccyx is the bottommost bone.