whip, rod, scourge, crime punishable by flogging
笞 centers on a tool for striking as punishment: a whip, rod, or scourge, and by extension the flogging itself.
笞 combines 竹 (bamboo) with 台, which likely contributes the sound. Bamboo rods were used for punishment, fitting the whip/rod meaning.
The top 竹 is bamboo, and the bottom 台 looks like a stand. Imagine a bamboo rod resting on a stand, ready to be used as a whip for punishment.
For チ, picture a whip made of bamboo striking with a sharp 'chi!' sound: chi -> チ, and the bamboo rod delivers the flogging.