The Collaborative International Dictionary of English v.0.48:

Pais \Pa`is\ (p[.a]`[-e]"), n. [OF. pu["i]s, F. pays, country.]
   (O. E. Law)
   The country; the people of the neighborhood.
   [1913 Webster]

   Note: A trial per pais is a trial by the country, that is, by
         a jury; and matter in pais is matter triable by the
         country, or jury.
         [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PAIS, or PAYS. A French word signifying country. In law, matter in pais is
matter of fact in opposition to matter of record: a trial per pais, is a
trial by the country, that is, by a jury.