Achilles' heel
Achil·les' heel
/əˈkıliz-/ noun , pl ⋯ heels [ count ]
: a fault or weakness that causes or could cause someone or something to fail
I'm trying to lose weight, but ice cream is my Achilles' heel.
This year, the team's Achilles' heel is its pitching. [=the team could fail because of its poor pitching]
