E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A Nisi Case, a cause to be tried in the assize courts. Sittings at Nisi Prius, sessions of Nisi Prius Courts, which never try criminal cases. Trial at Nisi, a trial before judges of assize. An action at one time could be tried only in the court where it was brought, but Magna Charta provided that certain cases, instead of being tried at Westminster in the superior courts, should be tried in their proper counties before judges of assize. The words Nisi Prius are two words on which the following clause attached to the writs entirely hinges:We command you to come before our justices at Westminster on the morrow of All Souls, NISI PRIUS justiciarii domini regis ad assisas capiendas venerinti.e. unless previously the justices of our lord the king come to hold their assizes at (the court of your own assize town).