The Collaborative International Dictionary of English v.0.48:

nun \nun\ (n[u^]n), n. [OE. nunne, AS. nunne, fr. L. nonna nun,
   nonnus monk; cf. Gr. ?, ?; of unknown origin. Cf. Nunnery.]
   [1913 Webster]
   1. A woman devoted to a religious life, who lives in a
      convent, under the three vows of poverty, chastity, and
      obedience.
      [1913 Webster]

            They holy time is quiet as a nun
            Breathless with adoration.            --Wordsworth.
      [1913 Webster]

   2. (Zool.)
      (a) A white variety of domestic pigeons having a veil of
          feathers covering the head.
      (b) The smew.
      (c) The European blue titmouse.
          [1913 Webster]

   Gray nuns (R. C. Ch.), the members of a religious order
      established in Montreal in 1745, whence branches were
      introduced into the United States in 1853; -- so called
      from the color or their robe, and known in religion as
      Sisters of Charity of Montreal.

   Nun buoy. See under Buoy.
      [1913 Webster]