The Collaborative International Dictionary of English v.0.48:

Traitorous \Trai"tor*ous\, a. [Cf. F. tra[^i]treux.]
   1. Guilty of treason; treacherous; perfidious; faithless; as,
      a traitorous officer or subject. --Shak.
      [1913 Webster]

   2. Consisting in treason; partaking of treason; implying
      breach of allegiance; as, a traitorous scheme.
      [1913 Webster] -- Trai"tor*ous*ly, adv. --
      Trai"tor*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

traitorously
    adv 1: in a disloyal and faithless manner; "he behaved
           treacherously"; "his wife played him false" [syn:
           faithlessly, traitorously, treacherously,
           treasonably, false]

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

TRAITOROUSLY, pleadings. This is a technical word, which is essential in an
indictment for treason in order to charge the crime, and which cannot be
supplied by any other word, or any kind of circumlocution. Having been well
laid in the statement of the treason itself, it is not necessary to state
every overt act to have been traitorously committed. Vide Bac. Ab.
Indictment, G 1; Com. Dig. Indictment, G. 6; Hawk. B. 2, c. 25, s. 55; 1
East's P. C. 115; 2 Hale, 172, 184; 4 Bl. Com. 307; 8 Inst. 15; Cro. C. C.
87; Carth. 319; 2 Salk. 683; 4 Harg. St. Tr. 701; 2 Ld. Raym. 870; Comb.
259; 2 Chit. Cr. Law, 104, note (b).