This word is an old fashioned word meaning not able to be trusted, not true to promises, deceitful or treacherous. Or my personal favorite deliberately faithless. The etymology of the word goes back to good 'ol Latin. If you break it down per can mean destruction of and fide is faith. What a loaded word. The word faith is one that people have much emotional investment around so to be without it is a grave character flaw bordering on soulless (I take literary license for usage of hyperbole here).
One of the definitions for this word I found enlightening was - "tending to betray; especially having a treacherous character as attributed to the Carthaginians by the Romans"
HISTORY LESSON ALERT:
Back in ~200-100 B.C. Carthage was a major world power and Rome was an up and coming one. In a series of three massive wars called the Punic Wars Rome took Carthage . It was so ravaged that most of any history from Carthage's point of view was destroyed. Therefore all that remains is the Roman's bashing of the Carthagians. They portrayed them as treacherous.
I thought that this falls in line perfectly with many world powers when they went to war would make their opponents seem as abhorrent as possible in order to rally the troops and gain support for the war effort.
For those of you that truly know me, you know that I'm not the worlds most historically knowledgable gal. But to say the least, this peaked my interest.
So remember kiddies, keep the faith full not less!