weak link

weak link idiom
The least dependable member of a group, as in The shipping department, slow in getting out orders, is our weak link in customer service, or They're all very capable designers except for Ron, who is clearly the weak link. This expression alludes to the fragile portion of a chain, where it is most likely to break. [Mid-1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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