from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Sports Vulnerability of a boxer to a knockout punch.
- n. Vulnerability, especially of a public figure, to destructive criticism.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A sensitive chin or jaw, the physical striking of which tends to result in a knockout.
An allusion to the fact that glass is easily breakable. (Wiktionary)