The Collaborative International Dictionary of English v.0.48:

Anonymous \A*non"y*mous\, a. [Gr. ? without name; 'an priv. + ?,
   Eol. for ? name. See Name.]
   Nameless; of unknown name; also, of unknown or unavowed
   authorship; as, an anonymous benefactor; an anonymous
   pamphlet or letter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

anonymous
    adj 1: having no known name or identity or known source;
           "anonymous authors"; "anonymous donors"; "an anonymous
           gift" [syn: anonymous, anon.] [ant: onymous]
    2: not known or lacking marked individuality; "brown anonymous
       houses"; "anonymous bureaucrats in the Civil Service"

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "anonymous":
   closet, cryptonymic, cryptonymous, incognito, inmost, innermost,
   innominate, interior, intimate, inward, isolated, nameless,
   personal, private, privy, retired, secluded, sequestered,
   unacknowledged, undefined, undesignated, unidentified, unknown,
   unnamed, unrecognized, unspecified, withdrawn, without a name



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ANONYMOUS. Without name. This word is applied to such.books, letters or
papers, which are published without the author's name. No man is bound to
publish his name in connexion with a book or paper he has published; but if
the publication is libellous, he is equally responsible as if his name were
published.