Perfidious

Adjective - literary - deceitful and untrustworthy: A perfidious lover.

"Five aces? You, sir, are a most perfidious scoundrel!"

This word is really fun to say, despite the Oxford dictionary's insistence that it is a literary word. I don't believe that any word should be labelled as merely literary because this could cause people to think that it is out of place in a spoken context. A word of this quality cannot be confined to books, especially when people are reading less 'literature' in favour of badly written blogs which make poor attempts at humour at the expense of innocent words. The dictionary does make up for its slanderous (great word) comment by giving a fantastic example of usage though.
Like other words I have written about in these pages, perfidious is somehow onomatopoeic. Yes, I know perfidy doesn't necessarily make a sound and therefore by definition cannot be onomatopoeic but don't you think perfidious sounds really perfidious?
It’s got some good derivatives as well: perfidiousness, perfidy and perfidiously.