According to Henshall, 刺 breaks down quite logically; リ represents “sword” or “to cut,” while 朿 means “thorn,” as well as “sharp” and “piercing.” Originally, 刺 meant “to stab with a sword.”
As for the shape of 朿, it combines 木 (tree) with Δ (a symbol of tapering). That symbol is a new one for me! The triangle gradually opened up into the 冂 shape now caught in the tree. So I suppose a tapered piece of wood is sharp and piercing.
We also see the 朿 shape in 策 (SAKU: policy, plan, whip). And as luck would have it, we recently saw this kanji inside 政策 (seisaku: policy, government + policy).
In case you’re wondering how a sharp piece of wood has anything to do with policies, Henshall says that 策 (which combines bamboo, 竹, with the sharp wood) was once a sharply tapered, thin piece of wood for beating. That is, it was a horsewhip. (Eek—they beat horses with wood?) He further says that “policy” got mixed up with all this by mistake. But it makes perfect sense to me—policies are like rules with which the government prods the populace into orderly behavior.
