become obstructed, pound (sterling, lb)
磅 has two distinct meanings that do not share one clear conceptual center: the idea of becoming obstructed or blocked, and the unit of weight pound (as in sterling or lb). The pound sense is a phonetic loan, while the obstruction sense relates to the stone component.
磅 combines 石 (stone) with 旁, which likely contributes the sound. The original meaning is probably related to the sound of stones crashing or something heavy, which extended to the pound weight. The obstruction sense may be a separate development or a borrowing.
The stone 石 on the left suggests something heavy or obstructive. The right side 旁 looks like a side view of a heavy weight. Together, 磅 can mean a heavy pound weight or something that obstructs like a stone.
For ボウ, imagine a heavy stone bowl (ボウ) that obstructs the path: bowl -> ボウ, and the stone bowl is as heavy as a pound.