The Collaborative International Dictionary of English v.0.48:

Rebuff \Re*buff"\ (r[-e]*b[u^]f"), n. [It. ribuffo, akin to
   ribuffare to repulse; pref. ri- (L. re-) + buffo puff. Cf.
   Buff to strike, Buffet a blow.]
   1. Repercussion, or beating back; a quick and sudden
      resistance.
      [1913 Webster]

            The strong rebuff of some tumultuous cloud.
                                                  --Milton.
      [1913 Webster]

   2. Sudden check; unexpected repulse; defeat; refusal;
      repellence; rejection of solicitation.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rebuff \Re*buff"\, v. t. [imp. & p. p. Rebuffed
   (r[-e]*b[u^]ft"); p. pr. & vb. n. Rebuffing.]
   To beat back; to offer sudden resistance to; to check; to
   repel or repulse violently, harshly, or uncourteously.
   [1913 Webster] re-build

WordNet (r) 3.0 (2006):

rebuff
    n 1: a deliberate discourteous act (usually as an expression of
         anger or disapproval) [syn: rebuff, slight]
    2: an instance of driving away or warding off [syn: rebuff,
       snub, repulse]
    v 1: reject outright and bluntly; "She snubbed his proposal"
         [syn: rebuff, snub, repel]
    2: force or drive back; "repel the attacker"; "fight off the
       onslaught"; "rebuff the attack" [syn: repel, repulse,
       fight off, rebuff, drive back]

Moby Thesaurus II by Grady Ward, 1.0:

204 Moby Thesaurus words for "rebuff":
   abjuration, abjure, abjurement, backfire, backlash, backlashing,
   bafflement, balk, be proof against, bear up, bear up against,
   beat back, beat off, block, boomerang, bounce, bound, brush,
   brush aside, brush off, brush-off, carom, challenge, chase,
   chase away, chase off, check, checkmate, chuck, chuck out,
   chucking, chucking out, cold shoulder, cold-shoulder,
   combative reaction, complaint, confusion, contemn, contempt,
   contradict, contradiction, contrecoup, counter, counteraction, cut,
   cut direct, declination, decline, declining, defeat, defiance,
   defy, demur, denial, deny, despisal, despise, despising,
   disapproval, disapprove, discard, disclaim, disclamation,
   discomfiture, discount, discounting, discouragement, disdain,
   dismiss, dismissal, disown, disownment, dispute, disregard,
   dissent, dissentience, drive away, drive back, endure, except,
   exception, exclude, exclusion, fend, fend off, flat refusal, foil,
   forswear, fractiousness, frustration, high-hat, hinder,
   hold at bay, hold off, hold out, hold up, humiliation, ignore,
   ignoring, keep at bay, keep off, kick, kickback, negativism,
   nonacceptance, nonapproval, nonconsideration, noncooperation,
   not receive, objection, obstinacy, obstruct, opposition, pack off,
   parry, pass by, pass up, passing by, passive resistance,
   point-blank refusal, protest, push aside, push back, put back,
   put-down, putting away, putting out, reaction, rebound, rebut,
   recalcitrance, recalcitrancy, recalcitration, recant, recantation,
   recoil, refractoriness, refusal, refuse, refuse to consider,
   refuse to receive, reject, rejection, reluctance, remonstrance,
   renitence, renitency, renounce, renouncement, repel, repellence,
   repellency, repercussion, repudiate, repudiation, repulse,
   repulsion, resilience, resist, resistance, reversal, reverse,
   revolt, ricochet, rout, scout, scouting, send away, send off,
   send packing, setback, short shrift, shove away, slight, sneer,
   sniff, snort, snub, spring, spurn, spurning, stand, stand up,
   stave off, stop, summary negative, the cold shoulder, the go-by,
   throw away, throw out, throwing out, thrust back, turn aside,
   turn away, turn away from, turn back, turn out, turning out,
   uncooperativeness, upstage, waive, ward off, withstand,
   withstanding