Untruthful means not telling the truth or being dishonest. When someone gives an untruthful answer, they're saying something they know isn't true, either by lying directly or by leaving out important facts that would change what others understand.

An untruthful person might tell their parents they finished their homework when they didn't, or claim they saw a movie they never watched to fit in with friends. Being untruthful is different from making an honest mistake: if you accidentally give someone wrong directions because you misremembered the street name, that's an error, not being untruthful. But if you deliberately give wrong directions as a prank, that's untruthful.

The word often appears in serious contexts. A witness in court must promise to give truthful testimony. A scientist who publishes untruthful research results damages their reputation and career. An untruthful campaign advertisement misleads voters.

Notice that untruthful describes both people and their statements: an untruthful statement is one that doesn't match reality, while an untruthful person is someone who regularly lies or deceives others. Once people discover you've been untruthful, rebuilding their trust can be extremely difficult.