"Perfidious Albion" is a hostile epithet for England or the United Kingdom: perfidious (Wiktionary) signifies one who does not keep his faith or word, while Albion is an ancient name for Britain

The use of the adjective "perfidious" to describe Britain has a long history; instances have been found as far back as the 13th century. Schmidt, H.D. 'The Idea and Slogan of "Perfidious Albion"'. Journal of the History of Ideas, Vol. 14, No. 4 (Oct., 1953), pp. 604-616. A very similar phrase was used in a sermon by the eminent seventeenth-century French bishop, preacher and theologian Jacques-Bénigne Bossuet:

The Catholic bishop's reference is to England's lack of loyalty to the Catholic faith: although England received the Catholic faith from Rome in the time of Pope Gregory the Great despite its isolation, since the Reformation it had become a Protestant country.

The coinage of the phrase in its current form, however, is conventionally attributed to Augustin, Marquis of Ximenez, a Frenchman who in a 1793 poem wrote:

which means "Let us attack perfidious Albion in her waters." In this context, Britain's perfidy was political: in the early days of the French Revolution many in Britain, the most liberal of the European monarchies, had looked upon the Revolution with mild favour, but following the overthrow and execution of Louis XVI, Britain had allied herself with the other monarchies of Europe against the Revolution in France. This was seen by the revolutionaries in France as a "perfidious" betrayal.

"La perfide Albion" became a stock expression in France in the 19th century, to the extent that the Goncourt brothers could refer to it as "a well-known old saying". It was utilised by French journalists whenever there were tensions between France and Britain, for example during the competition for colonies in Africa culminating in the Fashoda incident. The catchphrase was further popularised by its use in La Famille Fenouillard, the first French comic strip, in which one of the characters fulminates against "Perfidious Albion, which burnt Joan of Arc on the rock of Saint Helena" (This is, of course, a joke - carried away by his anti-English fury the character mixes up Joan of Arc with Napoleon, also a "victim" of England).