marking line, ascertain, assess, measure
絜 centers on precise measurement and evaluation: using a marking line to ascertain dimensions, assess something, or measure it accurately.
絜 combines 㓞 (a component associated with carving or cutting) with 糸 (thread), suggesting the idea of using a thread or line for precise marking or measurement. The exact historical development is uncertain.
The top 㓞 looks like a cutting tool, and the bottom 糸 is thread. Imagine using a knife and a taut thread as a marking line to measure and assess something precisely.
For ケツ, picture using a marking line to measure a piece of wood, then making a cut with a sharp "kets" sound: kets -> ケツ, as the thread snaps tight and the assessment is complete.