shaft of an arrow
簳 refers specifically to the shaft of an arrow, the long straight part between the tip and the fletching.
簳 combines 竹 (bamboo) with 幹 (trunk, stem), suggesting a bamboo shaft. The character likely developed as a specific term for an arrow shaft made from bamboo.
Bamboo 竹 on top of a trunk-like shape 幹: imagine a straight bamboo stalk being used as the shaft of an arrow.
For カン, picture a can of arrows with bamboo shafts: can -> カン, and each arrow has a straight 簳.