name of an affluent of the Poyang Lake
淝 is a rare kanji used primarily as a proper noun, specifically the name of a tributary of Poyang Lake. Its standalone meaning is tied to this geographical reference.
淝 combines 氵 (water) with 肥, likely as a phono-semantic compound where 肥 suggests the sound and water indicates a river name. The exact historical development is uncertain.
The water radical 氵 beside 肥 (fat/fertile) suggests a fertile river: picture the rich, fat waters of the Fei River flowing into Poyang Lake.
For ヒ, imagine the river named after a person called He (ヒ): He -> ヒ, and the river is his namesake.
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