Dishonesty means a lot more than just telling lies. A dishonest person will deceive people with more than words. To be dishonest is to lie, cheat, steal, or deceive, or mislead people in any way.
I could say that a dishonest person is someone who has chosen to be irrelevant to themselves and others. How can I trust a person who is known to lie? Why would I lend money or anything else I own if I suspect that they might steal? I can have no faith, confidence, or trust in such a person. To be dishonest is to be a hypocrite, to be false, to be irrelevant.
If you want to find out how honest a person is, try asking them difficult questions. Ask them a question about something that nobody can possibly know. An honest person will admit that they do not know. A dishonest person will either ignore you or change the subject.
People sometimes may say things that are false, but this does not always mean that they are a liar. They may actually believe that something is true or relevant even when it is not. This could be because they have been lied to. This is why I do not get angry with people who mistakenly say that they hate math or that a triangle has five sides. They may be honest but ignorant.