treacherous; disloyal; lying; untrustworthy
The perfidious Nazis first made a non-aggression treaty with Russia, and then invaded it anyway.
per (through)#fidere (to trust) (Latin)
per+FID+ious: FID stands for fidelity i.e. loyalty. Hence perfidious is its opposite Perfidious: imagine a guy who applies PERFume as well as DIOdrant. So he is disloyal to one of them.