felt, rug, carpet
毡 centers on a thick, pressed fabric made from animal hair: felt, and by extension the rugs and carpets made from it.
毡 combines 毛 (hair/fur) with 占, which likely contributes a phonetic element. The character originally referred to felt, a fabric made by matting animal hair.
The outer 毛 is hair or fur, and the inner 占 looks like a fortune-telling slip. Imagine pressing animal hair together under a fortune slip to make a thick felt rug.
For セン, picture a thousand (千, せん) strands of hair being pressed into a single felt mat: 千 -> セン, and all those hairs become the rug.
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