dishonest
Not telling the truth on purpose or trying to trick others.
To be dishonest means to deliberately lie, mislead, or hide the truth. A dishonest person might tell their parents they finished their homework when they didn't, or claim they broke a lamp by accident when they actually threw a ball at it on purpose.
Dishonesty isn't the same as being mistaken or confused. If you think the answer is 12 but it's actually 14, that's not dishonest: you genuinely believed you were right. But if you copy someone else's answer of 14 while pretending it's your own work, that's dishonest because you're deliberately deceiving your teacher.
Dishonesty takes many forms. A dishonest merchant might use rigged scales that show items weigh more than they actually do. A dishonest student might forge a parent's signature on a permission slip. Someone being dishonest might tell outright lies, leave out important facts, or give answers designed to mislead.
The opposite of dishonest is honest or truthful. When people discover you've been dishonest, they may lose trust in you, and rebuilding that trust takes a long time. That's why dishonesty damages relationships: once people doubt your word, they question everything you say, even when you're finally telling the truth.