The Collaborative International Dictionary of English v.0.48:

Flagitious \Fla*gi"tious\, a. [L. flagitiosus, fr. flagitium a
   shameful or disgraceful act, orig., a burning desire, heat of
   passion, from flagitare to demand hotly, fiercely; cf.
   flagrare to burn, E. flagrant.]
   1. Disgracefully or shamefully criminal; grossly wicked;
      scandalous; shameful; -- said of acts, crimes, etc.
      [1913 Webster]

            Debauched principles and flagitious practices. --I.
                                                  Taylor.
      [1913 Webster]

   2. Guilty of enormous crimes; corrupt; profligate; -- said of
      persons. --Pope.
      [1913 Webster]

   3. Characterized by scandalous crimes or vices; as,
      flagitious times. --Pope.

   Syn: Atrocious; villainous; flagrant; heinous; corrupt;
        profligate; abandoned. See Atrocious. --
        Fla*gi"tious*ly, adv. -- Fla*gi"tious*ness, n.
        [1913 Webster]

              A sentence so flagitiously unjust.  --Macaulay.
        [1913 Webster]

WordNet (r) 3.0 (2006):

flagitious
    adj 1: extremely wicked, deeply criminal; "a flagitious crime";
           "heinous accusations" [syn: flagitious, heinous]
    2: shockingly brutal or cruel; "murder is an atrocious crime";
       "a grievous offense against morality"; "a grievous crime";
       "no excess was too monstrous for them to commit" [syn:
       atrocious, flagitious, grievous, monstrous]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "flagitious":
   corrupt, criminal, degenerate, depraved, disgraceful, flagrant,
   glaring, gross, infamous, miscreant, nefarious, perverse, rotten,
   scandalous, shameful, sinful, vicious, villainous, wicked