The Collaborative International Dictionary of English v.0.48:

Infamous \In"fa*mous\, a. [Pref. in- not + famous: cf. L.
   infamis. See Infamy.]
   [1913 Webster]
   1. Of very bad report; having a reputation of the worst kind;
      held in abhorrence; guilty of something that exposes to
      infamy; base; notoriously vile; detestable; as, an
      infamous traitor; an infamous perjurer.
      [1913 Webster]

            False errant knight, infamous, and forsworn.
                                                  --Spenser.
      [1913 Webster]

   2. Causing or producing infamy; deserving detestation;
      scandalous to the last degree; as, an infamous act;
      infamous vices; infamous corruption. --Macaulay.
      [1913 Webster]

   3. (Law) Branded with infamy by conviction of a crime; as, at
      common law, an infamous person can not be a witness.
      [1913 Webster]

   4. Having a bad name as being the place where an odious crime
      was committed, or as being associated with something
      detestable; hence, unlucky; perilous; dangerous. "Infamous
      woods." --P. Fletcher.
      [1913 Webster]

            Infamous hills, and sandy perilous wilds. --Milton.
      [1913 Webster]

            The piny shade
            More infamous by cursed Lycaon made.  --Dryden.

   Syn: Detestable; odious; scandalous; disgraceful; base; vile;
        shameful; ignominious.
        [1913 Webster]

WordNet (r) 3.0 (2006):

infamous
    adj 1: known widely and usually unfavorably; "a notorious
           gangster"; "the tenderloin district was notorious for
           vice"; "the infamous Benedict Arnold"; [syn: ill-famed,
           infamous, notorious]

Moby Thesaurus II by Grady Ward, 1.0:

145 Moby Thesaurus words for "infamous":
   aberrant, abhorrent, abnormal, abominable, arrant, atrocious,
   awful, bad, base, beastly, beneath contempt, black, blackguardly,
   blamable, blameworthy, brutal, contemptible, corrupt, criminal,
   damnable, dark, degenerate, degraded, delinquent, deplorable,
   derogatory, despicable, detestable, deviant, dire, discreditable,
   disgraceful, disgusting, dishonorable, disreputable, dreadful,
   egregious, enormous, evil, execrable, fetid, filthy, flagitious,
   flagrant, foul, fulsome, grievous, gross, hardly the thing,
   hateful, heinous, horrible, horrid, ignoble, ignominious, illegal,
   improper, inappropriate, incorrect, indecorous, inglorious,
   iniquitous, knavish, lamentable, loathsome, lousy, low, miscreant,
   monstrous, nasty, naughty, nefarious, noisome, not done,
   not the thing, notorious, obnoxious, odious, off-base, off-color,
   offensive, opprobrious, out-of-line, outrageous, peccant, perverse,
   pitiable, pitiful, rank, rascally, recreant, regrettable,
   reprehensible, reprobate, repulsive, revolting, roguish, rotten,
   sacrilegious, sad, scampish, scandalous, schlock, scoundrelly,
   scurvy, seamy, shabby, shady, shameful, shameless, shocking,
   shoddy, sinful, sordid, sorry, squalid, stigmatized, terrible,
   too bad, unclean, undue, unfit, unfitting, unforgivable, unhealthy,
   unlawful, unpardonable, unpraiseworthy, unrespectable, unrighteous,
   unsavory, unseemly, unspeakable, unsuitable, unworthy, vicious,
   vile, villainous, wicked, woeful, worst, worthless, wretched,
   wrong, wrongful