Dishonest and deceptive

perfidious
(adj)
pǝr-FID-ee-ǝs
Meaning

treacherous; disloyal; lying; untrustworthy

1st Sample Sentence

The perfidious Nazis first made a non-aggression treaty with Russia, and then invaded it anyway.

Root

per (through)#fidere (to trust) (Latin)

Mnemonics

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.