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.
reed before the wind lives on, while mighty oaks do fall
Prov. An insignificant, flexible person is more likely not to get hurt in a crisis than a prominent or rigid person. Our office has new managers now; I plan to be as inconspicuous as possible while they reorganize everyone. A reed before the wind lives on, while mighty oaks do fall.
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."