Sports:

A pugilist possessed of a glass jaw is especially vulnerable to knock out punches, especially those to the chin.

Music:

Glassjaw is a post-hardcore band from Long Island, New York. I have no idea what that means, but I wish them luck.

Personality Flaw:

A person with a glass jaw is especially sensitive to destructive criticism, cannot take it in stride. This is usually characteristic of those who are very critical of others.