物差し
ものさし · monosashi · JLPT N2 · noun
ruler
📏 物差し = mono (thing) + sashi (measure); the thing you use to measure
Sashi comes from sasu, to put against or measure. A monosashi is simply a thing that measures length, so the word means ruler.
差 — 垂 (drooping) + 工 (work/tool)
Originally depicted a hand reaching down to grasp grain stalks of uneven length, suggesting discrepancy or unevenness. From this came the meaning 'difference' or 'to differ.' The reading chā specifically carries the sense of disparity, while other readings developed meanings like 'to dispatch' (chāi) and 'poor/lacking' (chà).