Studying possessive apostrophes I have searched on ELU the questions correlated to this matter.
I have found the following: «What is the correct possessive for nouns ending in s?» and «When did it become correct to add an 's' to a singular possessive already ending in 's'?»
Reading both the first one and the second one I am not able to understand* why "Achilles heel"† is correct, as we can read on Oxford Advanced Learner's Dictionary.
Precisely I am asking why we should write "Achilles heel" rather than "Achilles' heel".
Does it depend on etymological reasons strictly correlated to the Greek origin? if so, in different no mythological cases, should we write, for example, either "Achilles tendon" or "Achilles' tendon"?
* I am not able to understand the issue because there is no answer to my question.
† Named after the Greek hero Achilles. When he was a little child, his mother held him below the surface of the river Styx to protect him against any injury. She held him by his heel, which therefore was not touched by the water. Achilles died after being wounded by an arrow in the heel.