glass jawn.1. Sports Vulnerability of a boxer to a knockout punch.2. Vulnerability, especially of a public figure, to destructive criticism.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.glass jaw n (Boxing) boxing informal a jaw that is excessively fragile or susceptible to punches Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014glass? jaw? n. a person’s jaw, esp. that of a boxer, that is vulnerable to even a light blow. [1910?15] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.