The Collaborative International Dictionary of English v.0.48:

Indignant \In*dig"nant\, a. [L. indignans, -antis, p. pr. of
   indignari to be indignant, disdain. See Indign.]
   Affected with indignation; wrathful; passionate; irate;
   feeling wrath, as when a person is exasperated by unworthy or
   unjust treatment, by a mean action, or by a degrading
   accusation.
   [1913 Webster]

         He strides indignant, and with haughty cries
         To single fight the fairy prince defies. --Tickell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indignant
    adj 1: angered at something unjust or wrong; "an indignant
           denial"; "incensed at the judges' unfairness"; "a look of
           outraged disbelief"; "umbrageous at the loss of their
           territory" [syn: indignant, incensed, outraged,
           umbrageous]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "indignant":
   acrimonious, against, angered, angry, annoyed, browned-off, con,
   cross, disappointed, disapprobatory, disapproving, discontented,
   disenchanted, disgruntled, disillusioned, displeased, dissatisfied,
   dissenting, enraged, exasperated, furious, heated, huffy,
   in a huff, in a pet, in a rage, in a temper, incensed, irate,
   ireful, irked, irritated, livid, low, mad, miffed, opposed,
   opposing, peeved, piqued, pissed, pissed off, pissed-off, poor,
   provoked, riled, riled up, sore, ticked off, turned-off,
   unappreciative, unapproving, uncomplimentary, unfavorable, unhappy,
   vexed, worked up, wrathful, wrathy, wroth, wrought-up