foot, calyx, sitting in the lotus position
趺 centers on the foot, especially in a specific seated posture. It refers to the foot itself, the calyx (the foot-like base of a flower), and sitting in the lotus position where the feet are placed on the thighs.
趺 combines 足 (foot) with 夫, which likely contributes the sound. The character is used for the foot and for the lotus position, where the feet are placed upward.
The left side is 足 (foot). The right side 夫 looks like a person sitting cross-legged. Picture a foot in the lotus position: the foot is placed on the opposite thigh, forming a stable base like a calyx.
For シ, imagine a person sitting in the lotus position on a soft sheepskin: sheep -> シ, and the foot rests comfortably.