This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 歽 is uncertain. It combines 歹 (death-related) and 斤 (axe), but the original meaning and origin are unclear.
The left side 歹 suggests death or decay, and the right side 斤 is an axe. Picture an axe cutting something lifeless, but the overall meaning remains obscure.
For セツ, imagine a set of old, decayed axes: set -> セツ, and the axes are worn out like the deathly 歹 component.