If someone accuses you of being perfidious, you should probably be offended — it means underhanded, treacherous, deceitful — even evil.
If you betray people often, you're perfidious: traitors are extremely perfidious. Besides betrayal, this word implies lying and maybe other kinds of awful behavior, like stealing and taking bribes. Everyone tries to avoid perfidious people. Perfidious is kind of an old-fashioned word, but being perfidious will never go out of style, unfortunately.