sole of the foot
跖 refers specifically to the sole of the foot, the bottom surface that makes contact with the ground.
跖 combines 足 (foot) with 石 (stone), likely suggesting the part of the foot that touches the ground or a stone surface. The exact historical development is uncertain.
The left side 足 means foot, and the right side 石 means stone. Imagine the sole of your foot pressing against a flat stone—that's the part 跖 names.
For セキ, picture stepping barefoot on a stone and saying 'seki!' as a sharp exclamation: seki -> セキ, and the sole of your foot feels the stone.