Achilles' heel

When the hero Achilles was an infant, his sea-nymph mother dipped him into the river Styx to make him immortal. But since she held him by one heel, this spot did not touch the water and so remained mortal and vulnerable, and it was here that Achilles was eventually mortally wounded. Today, the tendon that stretches up the calf from the heel is called the Achilles tendon. But the term Achilles' heel isn't used in medicine; instead, it's only used with the general meaning "weak point"—for instance, to refer to a section of a country's borders that aren't militarily protected, or to a Jeopardy contestant's ignorance in the Sports category.