Who is your Achilles' tendon named for and why?

The origin lies in Greek mythology.

Achilles was the son of the nymph Thetis and Peleus, the king of the Myrmidons.

According to the Achilleid, written by Statius in the first century AD, when Achilles was born Thetis tried to make him immortal by dipping him in the river Styx. However, he was left vulnerable at the part of the body by which she held him, his heel.