at ease
媞 centers on a state of relaxed comfort and ease, often associated with a graceful or composed feminine presence.
媞 combines 女 (woman) with 是, which likely contributes the sound and may suggest correctness or propriety, together conveying a woman at ease or composed.
A woman 女 beside 是, which looks like a settled, balanced shape, suggests a woman resting comfortably at ease.
For シ, picture a woman at ease, softly saying 'she' as she relaxes: she -> シ.