flying fast, swooping
鴪 centers on rapid, downward bird flight: flying fast or swooping. The given meanings are two aspects of the same swift aerial motion.
鴪 combines 穴 (hole, cavity) with 鳥 (bird), likely suggesting a bird darting into or out of a hole at high speed. The exact historical development is uncertain.
A bird (鳥) shooting out of a hole (穴) like a feathered arrow: that sudden burst of speed is flying fast and swooping.
For イツ, picture a bird swooping so fast it makes a sharp 'eats!' sound as it snatches prey: eats -> イツ.
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