sickle, cut, carve, engrave
鍥 centers on a metal tool used for cutting or shaping: a sickle for reaping, or the act of carving and engraving with a sharp metal instrument.
鍥 combines 金 (metal) with 契, which likely contributes the sound and the idea of cutting or carving. The exact historical development is uncertain, but the metal radical points to a metal cutting tool.
The left side is metal 金, and the right side 契 suggests cutting or carving. Picture a metal sickle or engraving tool that carves into wood or stone.
For ケツ, imagine a metal sickle cutting through a thick stalk with a sharp 'kets' sound: kets -> ケツ, and the blade carves cleanly.