Untrustworthy describes someone or something you can't rely on or believe in. An untrustworthy person might promise to meet you after school but not show up, tell you things that aren't true, or say they'll help with a project and then disappear. When you call someone untrustworthy, you're saying their words and actions don't match up: they break promises, hide the truth, or let people down repeatedly.

The word can apply to things, too. An untrustworthy website might spread false information. An untrustworthy bridge might have rotten boards that could break. Scientists call data untrustworthy when it comes from sloppy experiments.

Notice that being untrustworthy is different from making a single mistake. Everyone forgets things sometimes or messes up accidentally. But an untrustworthy person shows a pattern: they consistently fail to keep their word or tell the truth. Once someone develops a reputation for being untrustworthy, rebuilding trust takes a long time. People start doubting everything they say, even when they're finally telling the truth.

The opposite is trustworthy: someone whose word you can count on, who shows up when they promise to, and who tells you the truth even when it's difficult.