Achilles' heel

Meaning: An Achilles' heel is a weakness that could result in failure.

For example:

• He's a good golfer, but his Achilles' heel is his putting and it's often made him lose matches.
  
• The country's dependence on imported oil could prove to be its Achilles' heel if prices keep on rising.
  

Origin: From the ancient Greek myth of Achilles whose mother made him invulnerable to attack by dipping him headfirst into a sacred river when he was a baby. She was holding him by one foot, and the heel of this foot wasn't touched by the water so it became his "weak point" later in life.