a weak link

Someone or something considered inferior to the other parts of a group, series, or mechanism. The weak link in computer security is almost always the end user. Derek hardly ever comes to class, so I'm not surprised he was the weak link in our group project.

COMMON If you describe someone or something as a weak link or a weak link in the chain, you mean that they are an unreliable part of a system or member of a group, and because of them the whole system or group may fail. It was automatically assumed that Edward would be the weak link in the partnership. Success comes from teamwork, and all it takes is one weak link in the chain to deny you the rewards of any amount of hard work. Note: People also say that someone or something is the weakest link if they are the most unreliable part of a system. He was the weakest link in the team's defence. Note: People sometimes say that a system is only as strong as its weakest link. A rail system is only as strong as its weakest link, as any commuter trapped behind a broken-down train can testify.