scissors, cut, clip, annihilate
翦 centers on cutting with a sharp tool: using scissors to clip or trim, and by extension annihilating or cutting down completely.
翦 combines 前 (before, front) and 羽 (feathers). The historical development is uncertain, but it may relate to cutting or trimming feathers, or 前 may contribute a phonetic element.
Imagine holding a pair of scissors (the cutting idea) in front (前) of a bird's feathers (羽), ready to clip them. The action of cutting feathers captures the core meaning of scissors and cutting.
For セン, picture a pair of scissors snipping a thousand (千, せん) feathers at once: 千 -> セン, and the scissors cut through them all.
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