water in which rice has been boiled
滫 refers specifically to the starchy water left over after boiling rice, a byproduct of cooking.
滫 combines the water radical 氵 with the phonetic component 脩, which likely contributes the sound and may relate to the idea of something steeped or processed. The exact historical development is uncertain.
The water radical 氵 beside 脩 suggests a liquid that has been steeped or processed. Picture the cloudy water left after boiling rice, with the starch making it look slightly thick.
For シュウ, imagine the sound of steam hissing as the rice water boils: 'shuuu' -> シュウ, and the starchy water is ready.