wash
頮 centers on washing, specifically the act of cleansing with water.
頮 combines 水 (water) and 頁 (head), suggesting washing the head or face. The bottom component 廾 may be a remnant of hands, reinforcing the action of washing.
Water 水 pours over a head 頁 while hands 廾 scrub: the whole scene is washing.
For カイ, imagine washing your face in a cool stream and exclaiming 'Kai!' as the water refreshes you: Kai -> カイ.
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