chink in (one's)/the armor

A minor but very detrimental flaw or weakness. Yeah, he's brilliant, but his violent temper has destroyed many business relationships—it's a real chink in his armor. The criminal's tendency to use his own cell phone to conduct business was the chink in the armor the police needed to put him in jail. A: "Why didn't George get the promotion?" B: Probably because he's such a hothead." A: "Good point. That really is a chink in his armor."

a chink in someone's armour

If someone or something has a chink in their armour, they have a weakness that people can take advantage of, although they seem very strong and successful. Note: `Armour' is spelled `armor' in American English. With their superior knowledge, they might find the chinks in his armour. Labour leaders hope to use their annual conference to attack what they currently see as the most vulnerable chink in the government's armour. Note: A chink is a small hole or opening.

a chink in someone's armour

a weak point in someone's character, arguments, or ideas which makes them vulnerable to attack or criticism.