Meaning that a person has a glass jaw, is easily incapacitated. Most commonly used to describe a large/intimidating individual who, upon entering in a fist fight, is knocked out after one or two punches. Originated from the slogan for the Ford Motor Company and the fact that Ford trucks look large and intimidating, but are prone to breakdowns, usually of the transmission.
John: Man, did you see that fight? Chris knocked Big Will out cold with one hit!
Charlie: What? Big Will's got a foot and a hundred pounds on Chris!
John: Well, I guess that Big Will is just Built Ford Tough