clear, bright and brilliant, sweat
泚 centers on clarity and brilliance, like clear water or a bright appearance. The sweat meaning is a specific application: clear, glistening sweat on the skin.
泚 combines the water radical 氵 with 此, which likely contributes the sound. The character expresses clear water or a bright, glistening appearance.
Water 氵 beside 此: imagine a clear stream where you can see every pebble, so bright it almost glistens like sweat on a hot day.
For セイ, picture a clear, bright day when you say 'say' to the sparkling water: say -> セイ, and the water is so clear it seems to speak.