care, chastity
愼 centers on mindful restraint: acting with careful attention and moral discipline, which includes the idea of chastity as a specific form of guarded conduct.
愼 combines 忄 (heart/mind) with 眞, which likely contributes both sound and the idea of truth or genuineness, suggesting a mind focused on true, careful conduct. The modern Japanese simplified form is 慎.
The heart 忄 beside 眞 (truth) suggests a mind that holds to what is true and pure, leading to careful, chaste behavior.
For シン, imagine a person practicing careful chastity while wearing a thin, sheer garment: thin -> シン, and the heart stays guarded.