stretch, extend
抻 centers on pulling something longer or wider: stretching or extending a physical object or a span.
抻 combines 扌 (hand) with 申, which likely contributes the sound and may suggest extending or stretching out. The exact historical development is uncertain.
The hand radical 扌 beside 申 looks like a hand pulling something long and straight, like stretching a piece of elastic.
For シン, imagine stretching a thin piece of fabric until it's almost transparent: thin -> シン, and your hand is doing the stretching.
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