“Se trouver entre le zist ct le zest.” To be in a quandary; in a state of perfect bewilderment. Also, to shilly shally. “Zest” is anything of no value, as “Cela ne vaut pas un zest” (It is not worth a fig). “Zist” is the same word slightly varied.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894