The Collaborative International Dictionary of English v.0.48:

D'ejeuner \D['e]`jeu`ner"\, n. [F. d['e]jeuner breakfast, as a
   verb, to breakfast. Cf. Dinner.]
   A breakfast; sometimes, also, a lunch or collation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dejeuner
    n 1: a midday meal [syn: lunch, luncheon, tiffin,
         dejeuner]

The Devil's Dictionary (1881-1906):

DEJEUNER, n.  The breakfast of an American who has been in Paris.
Variously pronounced.