The Collaborative International Dictionary of English v.0.48:

Nones \Nones\, n. pl. [L. nonae, so called because it was the
   ninth day before the ides, fr. nonus ninth, from novem nine.
   See Nine, Nones, 2, Noon .]
   [1913 Webster]
   1. (Roman Calendar) The fifth day of the months January,
      February, April, June, August, September, November, and
      December, and the seventh day of March, May, July, and
      October. The nones were nine days before the ides,
      reckoning inclusively, according to the Roman method.
      [1913 Webster]

   2. [F. none, fr. L. See Noon.] The canonical office, being
      a part of the Breviary, recited at noon (formerly at the
      ninth hour, 3 p. m.) in the Roman Catholic Church.
      [1913 Webster]

   3. The hour of dinner; the noonday meal. [Obs.]
      [1913 Webster]

            At my supper and sometimes at nones.  --P. Plowman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nones
    n 1: the fifth of the seven canonical hours; about 3 p.m.