Achilles' heel.

When Achilles was a little baby his mother Thesis took him to the River Stix. She wanted to protect Achilles from an early death. Thesis held her baby son's heel. She didn't think about this part of the body.

Many years later young Achilles went to Trojan War. He sustained an injury and died. So, Achilles' heel means weak place of something or somebody.

Source of image:

https://www.shutterstock.com/fr/image-illustration/legendary-grecian-warrior-wounded-by-arrow-279391916?src=glWEYC_YJEkpiyJhvv9L3Q-1-7