Perfidy is (OED): Deceitfulness, untrustworthiness; breach of faith or of a promise; betrayal of trust; treachery. These words reinforce how the prefix is used to mean "a completeness of' the thing that is attached. "Perfidy", however, does not mean 'complete faithfulness and trustworthiness'.

What does in mean as a prefix?

The prefix in, which means “in, on, or not,” appears in numerous English vocabulary words, for example: inject, influx, and insane.

Is per a prefix or root?

The English prefix per-, which means “through,” appears in hundreds of English vocabulary words, such as perish and person. You can remember that the prefix per- means “through” via the word permanent, for something that is permanent stays intact “through” the years.