Ask 10 people on the street to define knowledge. Do you really think that words which are made of sounds and letters have the exactness, for instance, of numbers?
The mere fact that the meanings of words do change, as per your own assertions, is a refutation of your claim that "knowledge," for instance, has an exact meaning.
Or do you think the meaning changes instantaneously?
No, words drift according to how they are used. And surely you have read enough philosophy to see how much this happens in the philosophical tradition alone. For instance, how would you define "philosophy"?
It's exactly because I have studied the evolution of language, and take a keen interest in metaphor, that I am reluctant to oversimplify. Derrida got famous for persuading quite a few people that language was just a system of differences, that words have meaning only in relation to other words. And the whole system slips now and then.