The Collaborative International Dictionary of English v.0.48:

Piarist \Pi"a*rist\, n. [L. pius pious.] (R. C. Ch.)
   One of a religious order who are the regular clerks of the
   Scuole Pie (religious schools), an institute of secondary
   education, founded at Rome in the last years of the 16th
   century. --Addis & Arnold.
   [1913 Webster]