thorn, brier, whip
荊 centers on a thorny plant used for punishment or hardship: a brier that pricks, and by extension a whip made from such thorny branches.
荊 combines the plant radical 艹 with 刑 (punishment), suggesting a thorny plant used for whipping or punishment. The exact historical development is uncertain, but the component relationship is clear.
The grass top 艹 covers 刑 (punishment). Imagine a thorny brier being used as a whip for punishment: the plant on top delivers the sting below.
For ケイ, picture a jailer named Kay whipping a prisoner with a thorny branch: Kay -> ケイ, and the whip is made of briers.