wedge, arrowhead
楔 centers on a wedge-shaped object used to split, tighten, or secure something. The arrowhead sense is a specific application of that same tapered, penetrating shape.
楔 combines 木 (tree/wood) with 契, which likely contributes the sound and the idea of cutting or carving. Together they suggest a wooden wedge or a tool for splitting.
A wooden 木 wedge 契 is driven into a log to split it. Picture the tapered shape forcing the wood apart.
For ケツ, imagine a wedge getting stuck and you need to give it a sharp 'kets' kick to drive it in: kets -> ケツ.
wedge; chock; linchpin; lynchpin
split wedge
to make sure (of); to drive in a wedge
to drive a wedge into (e.g. enemy defences); to wedge apart
wedge; lynchpin; cotter
cuneiform (writing)
slender sunfish (Ranzania laevis); oblong sunfish
wedge-shaped; cuneate; cuneiform
cuneiform (writing)
cuneiform (bones)
ice wedge
wedge (for securing objects together)