guard against, take care, caution
毖 centers on careful, preventive attention: guarding against harm, taking care, and exercising caution.
毖 is a phono-semantic compound: 比 (likely phonetic) over 必 (heart/mind), suggesting a cautious, guarded mindset. The exact historical development is uncertain.
Compare 比 (compare) above 必 (heart/mind): imagine carefully comparing two paths with your heart, guarding against the wrong choice.
For ヒ, picture a guard saying 'He!' to stop someone: 'He!' -> ヒ, a sharp sound of caution.