薄弱 (bóruò) describes a weak or inadequate spot in something abstract – defenses, foundations, or willpower – a vulnerability. It differs from 脆弱 (fragile, easily broken), 虚弱 (weak in health or strength), and 衰弱 (gradual decline in function). Use 薄弱 for the weakest link in a system, not for temporary physical illness.