straw raincoat
簔 centers on a traditional rain garment woven from straw, a straw raincoat.
簔 combines the bamboo radical 竹 with the phonetic component 衰, which likely contributed the sound and may relate to the idea of covering or wearing. The exact historical development is uncertain.
The top 竹 (bamboo) suggests woven plant material, and the bottom 衰 looks like a person wrapped in a protective covering. Picture a farmer wearing a straw raincoat made of bamboo sheaths.
For サ, imagine a straw raincoat keeping you dry during a sudden shower: the 'sa' in 'sudden' cues サ, and the raincoat protects you.