The Collaborative International Dictionary of English v.0.48:

Nonce \Nonce\ (n[o^]ns), n. [For the nonce, OE. for the nones, a
   corruption of for then ones, where n. in then is a relic of
   AS. m in [eth]am, dat. of the article and demonstrative
   pronoun, E. the. See For, Once, and The.]
   The one or single occasion; the present call or purpose; --
   chiefly used in the phrase

   for the nonce, i. e. for the present time.
      [1913 Webster]

            The miller was a stout carl for the nones.
                                                  --Chaucer.
      [1913 Webster]

            And that he calls for drink, I 'll have prepared him
            A chalice for the nonce.              --Shak.
      [1913 Webster]

   Nonce word, "a word apparently employed only for the
      nonce". --Murray (New English Dict.).
      [1913 Webster]

WordNet (r) 3.0 (2006):

nonce word
    n 1: a word with a special meaning used for a special occasion
         [syn: nonce word, hapax legomenon]