cut, file, trim
剉 centers on cutting or shaping by removing material: cutting, filing, or trimming something down.
剉 combines 坐 (sit) with 刂 (knife). The exact historical development is uncertain, but it likely expresses cutting or filing with a tool.
A knife 刂 sits beside 坐, like a file resting on a workbench while you trim and cut material.
For サ, imagine a saw cutting through metal: saw -> サ, and the knife 刂 files the surface smooth.