a person's jaw, especially that of a boxer, that is vulnerable to even a light blow.
Origin of glass jaw
First recorded in 1910–15
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for glass jaw
boxing informal a jaw that is excessively fragile or susceptible to punches
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