urine
溲 centers on urine, a bodily fluid. The given meaning is narrow and specific, with no clear broader conceptual cluster beyond this single sense.
溲 combines 氵 (water/liquid) with 叟, which likely contributes the sound. The character is used for urine, a liquid waste product.
The left side is water 氵, and the right side 叟 looks like an old man. Picture an old man needing to pass water: urine.
For シュ, imagine a shoe filled with urine: shoe -> シュ, and the water radical 氵 reinforces the liquid image.