Simple Definition of perfidious
: not able to be trusted : showing that someone cannot be trusted
Full Definition of perfidious
: of, relating to, or characterized by perfidy
Examples of perfidious
We were betrayed by a perfidious ally.
<a perfidious campaign worker revealed the senator's strategy to his leading rival for the nomination>
First Known Use of perfidious
Synonym Discussion of perfidious
Seen and Heard
What made you want to look up perfidious? Please tell us where you read or heard it (including the quote, if possible).