steal, secret, private, hushed
竊 centers on taking or doing something in a hidden, quiet way: stealing, acting secretly, keeping things private, or speaking in hushed tones.
竊 is the traditional form of 窃. It combines 穴 (hole, cavity) with 釆 (distinguish) and a right side that is a variant of 禼, which likely contributed the sound and the idea of stealth or hidden action. The modern simplified form 窃 replaces the complex lower part with 切.
The top 穴 suggests a hidden hole or cavity. Below, 釆 looks like a hand sorting or taking something, and the right side resembles a person moving quietly. Together, they create an image of secretly taking something from a hidden place.
For セツ, imagine a thief stealing a set of keys from a hidden hole: set -> セツ, and the thief acts secretly.