Lewis & Short

perfĭdĭōsus, a, um, adj. [perfidia], faithless, dishonest, treacherous, false, perfidious (class.).

  1. I. Of persons: fallax, perfidiosus, Cic. Pis. 27, 66: perfidiosus et subdolus animo, Tac. A. 16, 32.
    Sup.: omnium perfidiosissimus, Cic. N. D. 3, 32, 80.
  2. II. Of inanim. and abstr. things: amor, Plaut. Cist. 1, 1, 73: nihil perfidiosum et insidiosum et fallax in amicitiā, Cic. Fam. 3, 10, 7; id. Rosc. Com. 6, 16.
    Adv.: per-fĭdĭōsē, faithlessly, dishonestly, treacherously, perfidiously (rare but class.): hercle istam rem judicasti perfidiose, Plaut. Curc. 5, 3, 41: multa perfidiose facta, Cic. Rosc. Am. 40, 118: perfidiose, crudeliter factum, Auct. Her. 1, 5, 8.
    Comp.: perfidiosius rebellantes, Suet. Aug. 21.