flax, linen
紵 centers on plant fibers used for cloth: flax and the linen made from it.
紵 combines 糸 (thread) with 宁, which likely contributes the sound. The character refers to flax or linen thread.
The thread radical 糸 beside 宁 suggests a specific kind of plant fiber. Picture flax stalks being spun into linen thread.
For チョ, imagine a flax plant growing in a neat row, and you say 'cho!' as you pick the best one: cho -> チョ.
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