The Collaborative International Dictionary of English v.0.48:

Adieu \A*dieu"\, interj. & adv. [OE. also adew, adewe, adue, F.
   ? dieu, fr. L. ad to + deus God.]
   Good-by; farewell; an expression of kind wishes at parting.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Adieu \A*dieu"\, n.; pl. Adieus.
   A farewell; commendation to the care of God at parting.
   --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

adieu
    n 1: a farewell remark; "they said their good-byes" [syn:
         adieu, adios, arrivederci, auf wiedersehen, au
         revoir, bye, bye-bye, cheerio, good-by, goodby,
         good-bye, goodbye, good day, sayonara, so long]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "adieu":
   Godspeed, aloha, by, bye-bye, conge, doch-an-dorrach, farewell,
   good-bye, leave, leave-taking, parting, parting words, send-off,
   so long, stirrup cup, valediction, valedictorian, valedictory,
   valedictory address, viaticum