You are not to bear false witness against your neighbour.

Appearing as a false witness against one's neighbour or giving false evidence means in the strictest natural sense not being a false witness before a judge, or before other people out of court, against someone who is wrongly accused of some crime, and asserting this in God's name, or swearing by anything else holy, or by oneself, or by such things as affect the reputation of one's name. In a wider natural sense this commandment forbids all kinds of lying and hypocrisy in public life with evil intent; also, criticising and slandering one's neighbour so as to undermine his honour, name and reputation, on which the whole of a person's character depends. In the widest natural sense it includes using trickery, guile and deliberate wrong-dealing against someone for various causes, such as enmity, hatred, revenge, envy, rivalry, etc. These evil actions contain bearing false witness hidden within them.