plant allied to the water-lily
芡 refers to a specific plant related to the water-lily. Its meaning is narrow and botanical, with no clear abstract extensions.
芡 combines the plant radical 艹 with 欠, which may have contributed a phonetic element, but the exact historical development is uncertain.
The top grass radical 艹 suggests a plant, and the bottom 欠 looks like a person yawning or lacking something. Imagine a water-lily-like plant that seems to be missing a full bloom, hence its specific name.
For ケン, picture Ken (a person's name) yawning (欠) while looking at the water-lily plant: Ken -> ケン.
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