The term you may be referring to is "glass chin." "Glass chin" is a colloquial expression used in combat sports, particularly boxing, to describe a fighter who is susceptible to being knocked down or knocked out easily with a single punch to the chin. It suggests that the fighter's chin or jaw is weak or lacks resilience, making them more vulnerable to being hurt or knocked out by their opponents. It's worth noting that the term is not a medical or scientific diagnosis but rather a descriptive term used in the context of combat sports.