Mean your character is very dull

I think NERD is a proper word to decribe it. To be honest，we rarely use this phrase. Sometimes we use it (often say 你个木头脑袋，你个榆木疙瘩 rather than 你真木) to describe someone who reads a lot of books and has knowledge, but is being awkward in daily life like chatting with girls or just doesn’t know how to communicate properly with others. So I think NERD is a good explanation. If we want to say someone is dull，we often use 你真无聊。

you are numb. It means you cannot get someone's point immediately.

If someone says this to you and he or she is not happy,that means you are not smart and you can't get their point in the conversation or can't master the skill in doing something. And if someone says this to you in a very friendly way,especially when it comes to relationship,that means you are have to know more about the girl's thinking and be more active. Under this situation,"你真木" is usually told to a man.

If it was said by a girl, she may be flirting you, indicating that you bad boy can't understanding my true feeling.