What is the X in X-mas?
from Modern Latin tendo Achillis, first used by German surgeon Heister and so-called in reference to the one vulnerable spot of the great Greek hero Achilles, whose mother held him by the heel when she dipped him in the River Styx to render him invulnerable (though this story is not in Homer and not found before 1c. C.E.). Earlier Achilles' sinew, from Modern Latin chorda Achillis, coined 1693 by Dutch anatomist Philip Verheyden when dissecting his own amputated leg. Hence figurative use of heel of Achillies for "vulnerable spot" (1810).
Achilles tendon n.
The large tendon connecting the heel bone to the calf muscle of the leg. Also called calcanean tendon, heel tendon.