The Collaborative International Dictionary of English v.0.48:

Protestation \Prot`es*ta"tion\, n. [L. protestatio: cf. F.
   protestation. See Protest.]
   1. The act of making a protest; a public avowal; a solemn
      declaration, especially of dissent. " The protestation of
      our faith." --Latimer.
      [1913 Webster]

   2. (Law) Formerly, a declaration in common-law pleading, by
      which the party interposes an oblique allegation or denial
      of some fact, protesting that it does or does not exist,
      and at the same time avoiding a direct affirmation or
      denial.
      [1913 Webster]

WordNet (r) 3.0 (2006):

protestation
    n 1: a formal and solemn declaration of objection; "they
         finished the game under protest to the league president";
         "the senator rose to register his protest"; "the many
         protestations did not stay the execution" [syn: protest,
         protestation]
    2: a strong declaration of protest

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "protestation":
   affirmance, affirmation, alibi, allegation, announcement,
   annunciation, apology, assertion, asseveration, averment,
   avouchment, avowal, beef, bitch, blind, boycott, challenge, cloak,
   color, complaint, compunction, conclusion, cover, cover story,
   cover-up, creed, declaration, demonstration, demur, demurrer,
   device, dictum, enunciation, exception, excuse, expostulation,
   facade, feint, front, gloss, grievance, grievance committee, guise,
   handle, howl, indignation meeting, ipse dixit, kick, lame excuse,
   locus standi, manifesto, march, mask, nonviolent protest,
   objection, ostensible motive, picketing, poor excuse, position,
   position paper, positive declaration, predicate, predication,
   pretense, pretension, pretext, proclamation, profession,
   pronouncement, proposition, protest, protest demonstration,
   public motive, put-off, qualm, rally, refuge, remonstrance,
   remonstration, say, say-so, saying, screen, scruple, semblance,
   sham, show, sit-in, smoke screen, squawk, stalking-horse, stance,
   stand, statement, stratagem, strike, subterfuge, teach-in, trick,
   utterance, varnish, veil, vouch, word



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

PROTESTATION. An asseveration made by taking God to witness. A protestation
is a form of asseveration which approaches very nearly to an oath. Wolff,
Inst. Sec. 375.