Do Not Lie
Like sexuality, speech is a potent force in the world and can be the source of great harm if not done with awareness, integrity, and honesty. The Buddha likened the tongue to an axe. Words can harm others; words can harm you, even your own words towards yourself. This precept includes the four unwholesome actions of lying, speaking harshly, gossiping, and being frivolous.
This precept is much like the right speech of the Eightfold Path. Do not lie, slander, be dishonest in any way, speak with insincerity, promote falsehood, misrepresent information, or gossip maliciously. Do not be indifferent to the truth in any situation with any event that arises. Be truthful in everything you do and bring love and kindness into your environment.
The Buddha told his followers about the five courses of speech. Speech may be “timely or untimely, true or untrue, gentle or harsh, connected with good or with harm, or spoken with a mind of loving-kindness or with a mind of inner hate.” Obviously, he felt the skillful member of the five pairs to be the first.
Words are powerful, and the invitation here is to be mindful and intentional about how you use your words. In addition to not lying, you are encouraged to avoid words (including your own private thoughts) that are harsh, critical, angry, or belligerent. The Buddha also encouraged his followers not to gossip because of the harm it can cause.
American Buddhist teacher Jack Kornfield tells a story of making a commitment to not talk for a period of time about anything that was not about his direct experience (that is, gossiping about people he knew). He found, remarkably, that he had little to say!
The final consideration for speech is to make your speech meaningful and to avoid engaging in frivolous or useless talk. The ability to speak is such a wonderful capacity and one that may get taken for granted. If you remained silent instead of engaging in gossip and frivolous speech, how quiet would things get? The natural opening from these cautions is being receptive. When your mind is not entangled in lying, criticizing, gossiping, and wasting its time on frivolous talk, you could then enjoy a space for listening (and hopefully you'll be hearing the right speech of the person you are talking to!).