perfidious - purr-FID-ee-us - guilty of treachery or deceit
Example: (From Nietzche) "The most perfidious way of harming a cause consists of defending it deliberately with faulty arguments."
As a starter: and as a former teacher in English, I hope to add to reader's vocabulary by presenting a word a day in simple terms that anyone can understand - from current use in publications, radio,TV, etc. Remember: use a new word three times in a day and it's yours forever!