1. deliberately faithless; treacherous; deceitful:
The czarina was strong-minded, but neither cruel nor perfidious. (Jacques Casanova de Seingalt, The Memoirs of Casanova)
'You answer my questions - before you leave this spot I will know what you mean by this perfidious duplicity! (Anne Bronte, The Tenant of Wildfell Hall)
The most perfidious way of harming a cause consists of defending it deliberately with faulty arguments. (Friedrich Nietzsche)
Attaquons dans ses eaux la perfide Albion [Let us attack perfidious Albion in her waters]. (Augustin Louis de Ximenes, "L'Ère des Français", 1793) the first use of this stock expression which is still in use and has been given a gleeful linguistic boost, it seems, by Brexit, "the British will always be seen as the Perfidious Albion", Le Parisien, 2016.
Late 16th century: from Latin perfidiosus, from perfidia 'treachery'. (Oxford Dictionaries)