warn against, break, rend, tear
撕 centers on forceful separation: tearing, rending, or breaking something apart. The sense of warning against likely extends from the idea of tearing into someone verbally or breaking off a course of action.
撕 combines 扌 (hand) with 斯, which likely contributes the sound and the idea of splitting or dividing. The exact historical development of 斯 within this character is uncertain.
The hand radical 扌 beside 斯 suggests a hand forcefully pulling something apart. Imagine gripping a piece of cloth and tearing it with a sharp rending motion.
For セイ, picture tearing a piece of paper and saying "say" as you rip it apart: say -> セイ, and your hand does the tearing.
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