slender, beautiful body
娍 centers on a slender, beautiful body, describing a graceful physical form.
娍 combines 女 (woman) with 成, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound where 女 indicates the meaning relates to a woman's appearance.
A woman 女 who has achieved 成 a perfect, slender figure: 娍 is a beautiful, graceful body.
For セイ, imagine a slender woman saying 'say' as she models her beautiful form: say -> セイ.