(かさを～) さす
(かさを～) さす · (kasawo～) sasu · JLPT N2 · noun
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☂️ 傘を差す = umbrella + thrust up: kasa is umbrella, sasu is to raise it
差す (sasu) is the verb for pushing something up or in, which is why it also means a ray of light shining in. With an umbrella, you push the kasa up over your head, so 傘を差す means to open it or hold it up.