the.com/glass jaw

means A boxer's vulnerability to being knocked out by a single solid punch, regardless of their other skills or toughness.

from Straight out of boxing slang, likely early 20th-century American ring talk. The logic is blunt: glass looks fine until it meets a hard enough knock, then it doesn't crack politely — it shatters all at once. Applied to a fighter's chin, it captured the idea that some jaws absorb punishment while others fail catastrophically on the very first clean hit. The phrase jumped the ropes long ago and now covers anyone or anything — a sports team, an argument, a stock market — with one hidden, fatal weak point.