This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
妌 combines 女 (woman) with 井 (well). The exact historical development is uncertain, but it may have originally related to a woman's name or a quality associated with a well.
A woman 女 beside a well 井 suggests a scene of quiet, orderly fetching of water, but the kanji's meaning is not clearly defined.
For セイ, imagine a woman saying 'say' (セイ) something while drawing water from the well.