I've never heard or seen it as one word, but as "overly honest". It is a synonym of "tactless". It means saying the truth without taking into account the other person's feelings. An obvious example would be to tell a 300lb person that they are really too fat and that losing some weight would be much better for their health. Even though the person is telling the truth, it isn't helpful in any way and will most likely create problems in the relationship. Children are specialists at being overly honest and will say whatever comes to their mind. "Look mommy! That lady has a weird scar on her face!" Or "Daddy, why was mommy kissing that strange man the other day?"

'overhonest' is so honest that it may hurt someone.