fennel
茴 refers specifically to the plant fennel, an aromatic herb with feathery leaves and seeds used in cooking and medicine.
茴 combines the plant radical 艹 with 回, which likely provides the sound. The character was created to represent fennel, a plant whose name in Chinese sounded similar to 回.
The plant top 艹 sits above 回, like fennel fronds waving in a circular breeze. Think of fennel's round, ribbed seeds echoing the round shape of 回.
For ウイ, imagine a fennel plant swaying in the wind, making a soft 'wee' sound: wee -> ウイ. The feathery leaves whistle 'wee' as the wind circles around.
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