Perfidious. adjective: 1. deliberately faithless; treacherous; deceitful: a perfidious lover.
Quotes: “… that a brother should / Be so perfidious! ” — William Shakespeare, The Tempest, 1623
“‘Soft words will not work with me, for I know you only too well, perfidious knaves!’ said Don Quixote.” – Miguel de Cervantes (1547–1616), The Ingenious Hidalgo Don Quixote de la Mancha, translated by John Rutherford, 2000
Origin: Perfidious has an unexceptional history in English except for one curious phrase. The English adjective has always meant much the same as the Latin perfidiōsus “faithless, dishonest.” The unusual phrase is perfidious Albion or its French equivalent Albion perfide. (Albiōn is a Latin name for the island or nation of England). Perfidious entered English in the 16th century; perfidious Albion and Albion perfide in the 19th century (Albion perfide has been in French since the late 18th century).
Lexicography 