Psalms 34:14

נְצֹר לְשׁוֹנְךָ מֵרָע וּשְׂפָתֶיךָ מִדַּבֵּר מִרְמָה

Guard your tongue from evil and your lips from speaking deceit.

The concluding prayer at the end of Shemoneh Esrei begins with this verse from Psalms. We all know what lashon hara is – gossip and slander – but what is meant by deceit? The Radak explains that deceit refers to the things that two-faced people think. Such people plan to harm others while speaking to them as if they were really their friends.