Someone dishonest is corrupt: a cheater, a liar, or a fraud. Dishonest folks shouldn't be trusted.
If you know that being honest means to tell the truth and be straightforward, then you can probably guess that being dishonest means to tell lies and be crooked. Liars are dishonest. Criminals are dishonest. Some lawyers and politicians have a reputation for being dishonest, as do commercials that will say anything just to get you to spend your money. Besides people, certain actions are dishonest, like forging someone's signature. When you see the word dishonest, think "Untrustworthy!"