chisel for engraving, engrave
劂 centers on a specialized cutting tool: a chisel used for engraving, and by extension the act of engraving itself.
劂 combines 厥 (possibly phonetic) with 刂 (knife), suggesting a cutting or carving tool. The exact historical development is uncertain.
The left side 厥 looks like a complex block, and the right side 刂 is a knife. Imagine a knife carefully cutting into a hard block to engrave it: that's a chisel at work.
For ケツ, picture a craftsman using a chisel to engrave a stone, and with each strike he lets out a sharp 'kets!' sound: kets -> ケツ.