dregs
渣 centers on the leftover sediment or waste that settles after a liquid is processed: dregs, the undesirable remains.
渣 combines 氵 (water) with 查, which likely contributes the sound and the idea of inspecting or residue. The character suggests the dregs that remain after water is examined or processed.
Water 氵 on the left and 查 on the right: imagine inspecting a cup of tea and finding only the dregs left at the bottom.
For サ, picture the dregs settling at the bottom of a cup of sake (さけ): sake -> サ, and the leftover sediment is the dregs.
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