achillo-, achill-, Achilles
(Greek > Latin: tendon at the back of the heel)
The term "Achilles heel" was first used by a Dutch anatomist, Verheyden, in 1693 when he dissected his own amputated leg.
The Achilles tendon is one of the better known anatomic features in sports medicine because of the abrupt nature of some abrupt injuries to this tendon. It is in the back of the lower leg and connects the calf muscle (the triceps surae) with the calcaneus bone in the heel.
This tendon is so named for the hero whose heel, according to Greek mythology, was the only part of his body where he was unprotected from harm.
According to Greek mythology, achilles's mother held him as a baby by the heel and dipped him in the river styx to make him invulnerable but the heel by which she held him remained unimmersed and hence vulnerable, as Paris' arrow in Achilles' heel later proved.
See Achilles' Heel, the Myth for more details.