measuring unit, 1/16 of a ryou, percent, small, slight
銖 centers on a small unit of measurement, specifically one-sixteenth of a ryou. The meanings 'percent', 'small', and 'slight' all extend from this idea of a tiny fraction or amount.
銖 combines 金 (metal, gold) with 朱, which likely contributes the sound. The character originally referred to a small metal weight used as a unit of measurement.
The left side 金 (metal) suggests a metal weight, and the right side 朱 looks like a small tree or a tiny mark. Together they form a small metal weight used for measuring tiny amounts.
For シュ, imagine a tiny shoe made of metal that weighs exactly one 銖: shoe -> シュ, and the metal shoe is the small measuring unit.