This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
偗 combines 亻 (person) with 省, but the exact historical development and original meaning are uncertain.
A person 亻 beside 省, a shape associated with reflection or inspection, but the overall meaning is unclear.
For セイ, imagine a person saying 'say' while inspecting something: say -> セイ, and the person 亻 stands beside 省.