The Collaborative International Dictionary of English v.0.48:

Arrant \Ar"rant\, a. [OE. erraunt, errant, errand, equiv. to E.
   errant wandering, which was first applied to vagabonds, as an
   errant rogue, an errant thief, and hence passed gradually
   into its present and worse sense. See Errant.]
   Notoriously or pre["e]minently bad; thorough or downright, in
   a bad sense; shameless; unmitigated; as, an arrant rogue or
   coward.
   [1913 Webster]

         I discover an arrant laziness in my soul. --Fuller.
   [1913 Webster]

   2. Thorough or downright, in a good sense. [Obs.]
      [1913 Webster]

            An arrant honest woman.               --Burton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

arrant
    adj 1: without qualification; used informally as (often
           pejorative) intensifiers; "an arrant fool"; "a complete
           coward"; "a consummate fool"; "a double-dyed villain";
           "gross negligence"; "a perfect idiot"; "pure folly";
           "what a sodding mess"; "stark staring mad"; "a
           thoroughgoing villain"; "utter nonsense"; "the
           unadulterated truth" [syn: arrant(a), complete(a),
           consummate(a), double-dyed(a), everlasting(a),
           gross(a), perfect(a), pure(a), sodding(a),
           stark(a), staring(a), thoroughgoing(a), utter(a),
           unadulterated]

Moby Thesaurus II by Grady Ward, 1.0:

185 Moby Thesaurus words for "arrant":
   abject, abominable, absolute, atrocious, awful, bad, barefaced,
   base, beastly, beggarly, beneath contempt, black, blamable,
   blameworthy, blatant, bold, brassy, brazen, brazenfaced, brutal,
   cheesy, classical, complete, conspicuous, consummate, contemptible,
   crass, criminal, crummy, damnable, dark, debased, decided,
   definitive, degraded, deplorable, depraved, despicable, detestable,
   dire, dirty, disgraceful, disgusting, downright, dreadful,
   egregious, enormous, evil, execrable, fetid, filthy, flagitious,
   flagrant, flat-out, foul, fulsome, glaring, grave, grievous, gross,
   hanging out, hateful, heinous, horrible, horrid, improper,
   impudent, in relief, in the foreground, infamous, infernal,
   iniquitous, intolerable, knavish, lamentable, little, loathsome,
   lousy, low, low-down, lumpen, mangy, mean, measly, miserable,
   monstrous, nasty, naughty, nefarious, noisome, notable, noticeable,
   notorious, obnoxious, obtrusive, odious, offensive, ostensible,
   out-and-out, outrageous, outright, outstanding, overbold, paltry,
   peccant, perfect, petty, pitiable, pitiful, plain, poky, poor,
   positive, precious, profound, prominent, pronounced, proper, pure,
   rank, regrettable, regular, reprehensible, reprobate, reptilian,
   repulsive, rotten, sad, salient, scabby, scandalous, schlock,
   scrubby, scruffy, scummy, scurvy, shabby, shameful, shattering,
   sheer, shocking, shoddy, sinful, small, sordid, squalid, staring,
   stark, stark-staring, sticking out, striking, superlative,
   surpassing, terrible, the veriest, thorough, thoroughgoing,
   too bad, total, unabashed, unbearable, unblushing, unclean,
   unconscionable, undeniable, unequivocal, unforgivable,
   unmentionable, unmitigated, unpardonable, unqualified, unrelieved,
   unspeakable, unspoiled, unworthy, utter, vicious, vile, villainous,
   wicked, woeful, worst, worthless, wretched, wrong