clover, medic
蓿 refers to clover and similar medic plants, likely used as fodder or ground cover.
蓿 combines the grass radical 艹 with 宿, which likely provides the sound. The character is used for clover-like plants, but its detailed historical development is uncertain.
The grass radical 艹 sits above 宿, like clover growing around a lodging. Imagine a traveler resting at an inn surrounded by clover.
For シュク, picture a clover patch near a school (宿題, しゅくだい, homework) where students rest: 宿 -> シュク, and the clover grows right outside.
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