The Collaborative International Dictionary of English v.0.48:

Felonious \Fe*lo"ni*ous\, a.
   Having the quality of felony; malignant; malicious;
   villainous; traitorous; perfidious; in a legal sense, done
   with intent to commit a crime; as, felonious homicide.
   [1913 Webster]

         O thievish Night,
         Why should'st thou, but for some felonious end,
         In thy dark lantern thus close up the stars? --Milton.
   -- Fe*lo"ni*ous*ly, adv. -- Fe*lo"ni*ous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

felonious
    adj 1: involving or being or having the nature of a crime; "a
           criminal offense"; "criminal abuse"; "felonious intent"
           [syn: criminal, felonious]

Moby Thesaurus II by Grady Ward, 1.0:

86 Moby Thesaurus words for "felonious":
   actionable, against the law, amoral, anarchic, anarchistic, anomic,
   black-market, bootleg, chargeable, conscienceless, contraband,
   contrary to law, corrupt, corrupted, criminal, crooked, dark,
   devious, dishonest, dishonorable, doubtful, dubious, evasive,
   evildoing, fishy, flawed, fraudulent, ill-got, ill-gotten, illegal,
   illegitimate, illicit, immoral, impermissible, indirect,
   iniquitous, insidious, irregular, justiciable, lawless,
   malefactory, malfeasant, nonconstitutional, nonlegal, nonlicit,
   not kosher, outlaw, outlawed, punishable, questionable, rotten,
   shady, shameless, shifty, sinful, sinister, slippery, suspicious,
   triable, tricky, unallowed, unauthorized, unconscienced,
   unconscientious, unconscionable, unconstitutional,
   under-the-counter, under-the-table, underhand, underhanded,
   unethical, unlawful, unofficial, unprincipled, unsavory,
   unscrupulous, unstatutory, unstraightforward, unwarrantable,
   unwarranted, wicked, without remorse, without shame, wrong,
   wrongdoing, wrongful