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a broken reed
An unreliable or unsupportive person. I thought I could count on my best friend for support during this difficult time, but she proved to be a broken reed and never returned my calls.
an unreliable or undependable person. (On the image of a useless, broken reed in a reed instrument.) You can't rely on Jim's support. He's a broken reed. Mr. Smith is a broken reed. His deputy has to make all the decisions.
A weak or unreliable support, as in I'd counted on her to help, but she turned out to be a broken reed. The idea behind this idiom, first recorded about 1593, was already present in a mid-15th-century translation of a Latin tract, "Trust not nor lean not upon a windy reed."