presentable woman
姃 centers on a woman who is presentable, proper, or upright in appearance and conduct.
姃 combines 女 (woman) with 正 (correct, upright), suggesting a woman of proper or upright character. The exact historical development is uncertain, but the component relationship is straightforward.
A woman 女 beside 正 (correct, upright) is a woman who is proper and presentable.
For セイ, imagine a presentable woman saying 'say' with perfect poise: say -> セイ, and her upright manner makes her presentable.