脆弱 (cuì ruò) means fragile, weak, easily broken, damaged, or emotionally hurt — used for bodies, objects, and mental/emotional states. 薄弱 (bó ruò) focuses on weakness as insufficiency or lack of strength (foundations, defenses, willpower), while 虚弱 (xū ruò) is about poor physical health from illness. The one-character 脆 (cuì) means crisp/brittle texture or clear sound, not abstract fragility.