Perfidious comes from the Latin adjective perfidiosus (meaning treacherous or faithlessness), derived from perfidia (faithlessness) and perfidus (faithless).  It combines the prefix per- (meaning "through" or, in this destructive sense, "to ill effect" / "away from the bad") with fides or fidem, meaning "faith," "trust," or "fidelity". Literally, it describes breaking faith or deceiving through trust.  

According to the Oxford English Dictionary, the adjective perfidious was first used in English in the mid-1500s (before 1538), appearing in the writing of Wilfrid Holme.  William Shakespeare helped popularize the term in early modern English literature, such as in his play All's Well That Ends Well.  

The phrase "perfidious Albion" was first coined in French (la perfide Albion) by the French poet and playwright Augustin Louis de Ximénès in 1793, during the early French Revolutionary Wars.  "Albion" is an ancient poetic name for Britain, derived from the white cliffs of Dover.  The term reflected French feelings of betrayal when Britain shifted from a neutral or supportive stance during the French Revolution to joining forces with other European monarchies against France.  The phrase became a common stock expression in France during the 19th century—particularly popularized during the Napoleonic Wars—and was later used whenever political tensions flared between Britain and France.