duplicity

We’ve all probably dealt with someone who acted a little two-faced—they said one thing and did another, for example, or they talked “from both sides of their mouth.” If such behavior has made you do a double take or left you feeling double-crossed, you may be single-minded in your quest to learn more about duplicity. Duplicity comes from a long line of “double” talk, starting with its Latin ancestor duplex, which means “double” or “twofold.” Duplex is also the source of the English word duplex (which can be a noun meaning “a two-family house” or an adjective meaning “double”), and it is the root of another term for doubling it up, duplicate.

The Double Nature of Duplicity and Duplicitous

The idea of doubleness is at the core of duplicity and duplicitous. Duplicity is the older of the pair; it comes from a Latin word meaning "double" or "twofold," and its original meaning in English has to do with a kind of deception in which you intentionally hide your true feelings or intentions behind false words or actions. If you are being duplicitous there are two yous: the one you're showing and the one you're hiding. And—key to the idea of duplicity—you're hiding the hidden you in order to make people believe something that's not true.

The word is found in many works of literature, including the Bible:

The integrity of the upright guides them, but the unfaithful are destroyed by their duplicity.
— Proverbs 11:3 (New International Version)

Even when he sat opposite me in the kitchen, talking, he would turn his head a little toward the clock or the stove and look at me from the side, but with frankness and good nature. This trick did not suggest duplicity or secretiveness ...
— Willa Cather, My Antonia, 1918

Arch read it at breakfast the day it came out and again that night. He admired its art but was most affected, and in fact discomfited, by its unblinking inventory of self-seeking and duplicity.
— Tobias Wolff, Old School, 2003