from the GNU version of the Collaborative International Dictionary of English
- conj. unless before; -- a phrase applied to terms of court, held generally by a single judge, with a jury, for the trial of civil causes. The term originated in a legal fiction. An issue of fact being made up, it is, according to the English practice, appointed by the entry on the record, or written proceedings, to be tried by a jury from the county of which the proceedings are dated, at Westminster, unless before the day appointed (nisi prius) the judges shall have come to the county in question (which they always do) and there try the cause. See In banc, under Banc.
from The Century Dictionary and Cyclopedia
- A phrase occurring originally in a writ by which the sheriff of a county was commanded to bring the men impaneled as jurors in a civil action to the court at Westminster on a certain day, ‘unless before’ that day the justices came to the county in question to hold the assizes, which they were always sure to do.
Sorry, no etymologies found.