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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

FELONIOUSLY, pleadings. This is a technical word which must be introduced
into every indictment for a felony, charging the offence to have been
committed feloniously; no other word, nor any circumlocution, will supply
its place. Com. Dig. Indictment, G 6; Bac. Ab. Indictment, G 1; 2 Hale, 172,
184; Hawk. B. 2. c. 25, s. 55 Cro. C. C. 37; Burn's Just. Indict. ix.;
Williams' Just. Indict. iv., Cro. Eliz. 193; 5 Co. 121; 1 Chit. Cr. Law,
242.