The Collaborative International Dictionary of English v.0.48:

Felon \Fel"on\, n. [OE., adj., cruel, n., villain, ruffian,
   traitor, whitlow, F. f['e]lon traitor, in OF. also, villain,
   fr. LL. felo. See Fell, a.]
   1. (Law) A person who has committed a felony.
      [1913 Webster]

   2. A person guilty or capable of heinous crime.
      [1913 Webster]

   3. (Med.) A kind of whitlow; a painful imflammation of the
      periosteum of a finger, usually of the last joint.

   Syn: Criminal; convict; malefactor; culprit.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Felon \Fel"on\, a.
   Characteristic of a felon; malignant; fierce; malicious;
   cruel; traitorous; disloyal.
   [1913 Webster]

         Vain shows of love to vail his felon hate. --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

felon
    n 1: someone who has committed a crime or has been legally
         convicted of a crime [syn: criminal, felon, crook,
         outlaw, malefactor]
    2: a purulent infection at the end of a finger or toe in the
       area surrounding the nail [syn: felon, whitlow]

The Devil's Dictionary (1881-1906):

FELON, n.  A person of greater enterprise than discretion, who in
embracing an opportunity has formed an unfortunate attachment.