or Achilles' heel
a portion, spot, area, or the like, that is especially or solely vulnerable: His Achilles heel is his quick temper.
Origin of Achilles heel
First recorded in 1800–10
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for achilles heel
a small but fatal weakness
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012