Achilles' heel.
Achilles' heel.
'Achilles' heel' refers to a person's single, often fatal, point of vulnerability. Originating from the Greek mythology where Achilles was invulnerable except for his heel, this idiom is used in modern contexts to describe the one weakness in an otherwise strong individual or system, pointing out that everyone or everything has a vulnerable spot that could lead to their downfall if exploited.
Procrastination is his Achilles' heel.
Procrastination is his main weakness.
She excelled in all subjects, but math was her Achilles' heel.
She excelled in all subjects, but she struggled with math.
Public speaking is his Achilles' heel despite being knowledgeable.
Despite being knowledgeable, public speaking is his major weakness.