əkɪliːzˈhiːl/

noun: Achilles heel; plural noun: Achilles heels; noun: Achilles’ heel; plural noun: Achilles’ heels

a weakness or vulnerable point.
synonyms: weak spot, weak point, weakness, vulnerable spot, soft underbelly, shortcoming, failing, imperfection, flaw, defect, deficiency, fault, foible, chink in one’s armour;