But how can you convey truth if you don't really understand it? And how can you convey truth even if you understand it, if you don't understand what the listener understands when you speak of it?
It's not enough to use "correct" words, if they won't be understood. That's not honesty. That's how clever people take advantage; which is not honesty.
In these terms, I've never met anyone who seems like they could be entirely honest, even if they wanted to be.
But they can learn how to convey something closer to truth with practice and introspection and learning from others; so I'd say it's a skill.
I've met people who say they are honest, but a few minutes listening and from the inconsistent thinking it is apparent they are not even able to be honest within their own thoughts to themselves.
This is not a criticism because I think it applies to everyone, including myself. I think it's just part of the human condition.
To say a person is honest, then, is to say they desire to be honest; that their heart is in it, that they wish to convey truth and not to decieve, or even to risk misunderstandings, no matter who is listening. As noble as it is to desire this, it is quite something to master the art of doing so regularly and reliably.