Noob spelled backwards.
Although it does not mean the opposite of 'Noob', it has to do with the same idea. While a noob is someone lacking experience, therefore tey are rather terrible at whatever it may be, a boon is somebody with experience but for whatever reason, they are just as bad. A boon plays at the skill level of a noob but they do't have an excuse because they have experience in the field they are performing in.
Mike: John, you're such a boon.
John: What are you talking about? I just started playing.
Mike: You've been playing for 2 years and you are still terrible!