an ugly woman
媸 centers on ugliness, specifically describing an ugly woman. The meaning is narrow and concrete, without clear abstract extensions.
媸 combines 女 (woman) with 蚩, which likely contributes the sound and carries a sense of ugliness or foolishness. The exact historical development is uncertain.
A woman 女 beside 蚩, a shape that suggests something unpleasant or ugly. Picture a woman whose appearance is marred by the ugly, insect-like 蚩 on her right.
For シ, imagine an ugly woman who is so unpleasant that people say 'She!' with a hiss of distaste: She -> シ.