to irrigate, to flood water flowing, to scour
摡 centers on moving water forcefully over something: irrigating fields, flooding an area, or scouring a surface clean with flowing water.
摡 combines 扌 (hand) with 既, which likely contributes the sound and may relate to the idea of completion or thoroughness, suggesting a hand action that thoroughly applies water.
The hand radical 扌 beside 既 suggests a hand directing water: imagine a farmer's hand opening an irrigation gate, letting water flood and scour the fields.
For カイ, picture a hand opening a sluice gate with a loud 'kai' sound as the water rushes out to irrigate: kai -> カイ.