A·chil·les' heel /əˌkɪliːz ˈhiːl/ BrE AmE noun [countable]
[ Date: 1800-1900 ; Origin: From the story that the ancient Greek hero Achilles was dipped as a baby into the river Styx to protect him, but the part of his heel he was held by did not get wet, and so remained unprotected ]
a weak part of someone’s character, which could cause them to fail at something:
I think Frank’s vanity is his Achilles' heel.