This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 乴 is uncertain. It visually combines 折 and 乙, but the original component relationship is unclear.
Since the meaning is unclear, focus on the shape: 折 (fold/break) over 乙 (a hook). Imagine a broken hook that cannot be used.
For セツ, imagine a set of broken hooks: set -> セツ, and the broken hook shape matches the kanji.