Douglas Harper's Etymology Dictionary
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).
Wiktionary
n. (context anatomy English) The strong tendon formed of the united tendons of the large muscles in the calf of the leg, an inserted into the bone of the heel.
Wikipedia
The Achilles tendon or heel cord, also known as the calcaneal tendon , is a tendon of the back of the leg, and the thickest in the human body. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus (heel) bone. These muscles, acting via the tendon, cause plantar flexion of the foot at the ankle, and flexion at the knee.
The tendon can rupture and become inflamed.
" Achilles heel", referring to a vulnerability, relates to the mythical story of Achilles, who was slain during the Trojan war by a poisoned arrow to his heel.
Usage examples of "achilles tendon".
A month later he sprained it again, this time tearing some tissue around the Achilles tendon.
Yet when she pinched Nancy Greenly's Achilles tendon, there was no response no matter how hard she squeezed.
The razor steel sliced through the back of Le Riche's boot, and there was an audible pop as it severed his Achilles tendon.
Perennius drew his knife through the soft leather and the Goth's Achilles tendon, hamstringing the big man.
It's better to be conservative and take what you can get than take a big gamble and blow it, so Enzo reaches in, even as Raven is looking down at him, and severs Raven's left Achilles tendon.
They were intentionally built with weak motors, so even if the sensitive bumper bar at the front failed he'd still get nothing more than a bad bruise, but it was unsettling how they were managing to hit him squarely on the Achilles tendon.
He walked with a lilting gait, his left Achilles tendon apparently shortened, pulling his left heel up.
Sarah Perlan, a short and portly woman, wanted to sue the local tennis center for the Achilles tendon she'd torn when she'd stumbled on a wayward ball.
It bounded toward the combatants like a wolf running and set its jaws above Meder's heel where the Achilles tendon would be in a man.