Perfidious

1590s, from Latin perfidiosus "treacherous," from perfidia

Full definition of perfidious

Of, pertaining to, or representing perfidy; disloyal to what should command one's fidelity or allegiance.

1610, , by William Shakespeare, act 2 scene 2TRINCULO (speaking about Caliban): By this light, a most perfidious and drunken
monster: when his god's asleep, he'll rob his bottle.
1851, Oliver Goldsmith, Dr. Goldsmith's History of Rome (ed. William C. Taylor), ch. 26:The perfidious Ricimer soon became dissatisfied with Anthe'mius, and raised the standard of revolt.
1905, Andrew Lang, John Knox and the Reformation, ch. 14:She knew Huntly for the ambitious traitor he was, a man peculiarly perfidious and self-seeking.
2005 June 21, Robert Hughes (critic), "Art: The Velocipede of Modernism," Time:When the Nazis branded Feininger a "degenerate artist" in 1937, he left 54 paintings for safekeeping with a Bauhaus friend named Hermann Klumpp. After the war, and for the rest of Feininger's life, the perfidious Klumpp refused to give them back.