|Statue of Achilles|
Thetis tried to make her young son immortal by dipping him in the river Styx, As she immersed him, she held him by one heel and forgot to dip him a second time so the heel she held could get wet too. Therefore, the place where she held him remained untouched by the magic water of the Styx and that part stayed mortal or vulnerable. His heel remained dry and became the vulnerable spot where Achilles ultimately received his death-wound (at the hands of Hector's brother, Paris, who had help from the god Apollo).
To this day, any weak point is called an “Achilles’ heel”.