When describing an idiot, "一根弦的傻子"?
As Pedroski has explained, 一根弦的傻子 or 脑子一根弦 refers to those who clings to one idea. 一根弦 can be interpreted as &quot;single-minded&quot;. But note that it is not necessarily a derogatory term.
For example, in the following sentence,

一根弦的傻子，也能熬出头。
A single-minded fool can eventually survive from the nights and succeed as well.

一根弦 implies that the &quot;fool&quot; is determined and strong-willed and will not give up on what he/she has been pursuing for.