The Collaborative International Dictionary of English v.0.48:

Xyst \Xyst\, Xystus \Xys"tus\, n. [L. xystus, Gr. ?, from ? to
   scrape, polish; -- so called from its smooth and polished
   floor.] (Anc. Arch.)
   A long and open portico, for athletic exercises, as
   wrestling, running, etc., for use in winter or in stormy
   weather.
   [1913 Webster]