They make dictionaries for lay definitions in common usage. Once again, you won't find medical, scientific, engineering, theological or many other specific terminologies in it, as well as altered definitions from those technical usages, because the dictionary isn't a technical book. If you just go by the dictionary, you won't even know there is a difference between 'agnostic' and 'ignostic,' and yet 'ignostic' is a real term with meaning.
But once again, even the dictionary doesn't agree with you. It seperates belief and knowledge into two categories. A/theism deals with belief and a/gnostic deals with knowledge. Since they deal with two different things without overlap, they are not mutually exclusive.