Someone who is perfidious breaks a trust on purpose. This is not just 'bad' or 'dishonest' — the word needs a promise, a friendship, a side, an oath, something the person was supposed to be loyal to. A stranger who cheats you is not perfidious; a friend who cheats you is. That is the whole shape of the word: the betrayal comes from inside, from someone who was let in. It is a very formal, literary, old-fashioned word. You will meet it in novels, in history books ('the perfidious king'), in speeches, in a lawyer's angry letter — almost never in speaking. If a friend told you 'my ex was perfidious', you would smile, because it sounds like a stage play. In real speech people say 'he betrayed me', 'she stabbed me in the back', 'two-faced', 'disloyal'. It usually goes in front of a noun (a perfidious ally, perfidious behaviour) and it carries real moral anger — the writer is not describing, the writer is condemning. English speakers who know history often think of 'perfidious Albion', an old French insult for Britain, meaning a country that signs agreements and then quietly does the opposite. Things as well as people can be called perfidious in writing, but only things tied to a person's choices: a perfidious plan, a perfidious promise.

背信的、不忠的、背叛的（很正式的書面語，帶強烈譴責）