All dictionaries are based on "common usage," not on arbitrary authority. Common usage is how words originally obtained their meanings, long before humans even had a written language. Dictionaries are just a list of the common meaning of words--"common" in this sense meaning what we mean "in common" with other people who speak the same language. That's how language evolves, and is why the English of 500 or even 100 years ago differs from what we speak today, and why we Americans and the British are separated by a common language.
When Eileen and I were hunting in Ireland five years ago, we started talking about computers and customer service with our Irish hosts. We mentioned how often our service calls were taken by somebody who didn't even speak English as a primary language.
"Oh, it's the same way here!" our friend Liam said. "Our calls go to Scotland, and you can hardly understand a sentence. All full of bloody 'ochs'!"
