Dictionaries are descriptive, not prescriptive. A society, culture, or group of language users collectively decide how a word is to be used or defined, and then a dictionary reflects that usage. It's in the dictionary because it's how we use it, we don't use it a particular way because it's in the dictionary. This is how new words get added every year and how "non-words" become "words" - such as when "D'oh" was actually added to the dictionary thanks to one Homer J. Simpson. It certainly existed as an exclamation before him, but only after him did it come into popular usage to point where dictionaries added entries. We use the dictionary as a reference tool to discover or clarify how people use words, but again, the dictionary only describes how we use them, it does not prescribe how we should use them. I.e. dictionaries are not the final arbiter of usage, the collective group of language users are.