OSI didn't invent the term, and weren't the first ones to use it.

Why shouldn't they be the ones required to "make up their own word"? "Open source" has a reasonable plain english meaning which doesn't automatically include all the requirements of OSI's "definition". I don't see why the simplest plain english interpretation shouldn't be considered "the real one".

You say OSI's definition is "the widely accepted one", I'm 100% certain that's not true if your sample group is representative of "english speakers" instead of "hacker news readers".

(And I'm pretty sure your linked xkcd supports my argument here more than yours.)

Your response is a bit ironic as OSI more or less did invent the term (causality-wise, rather much of the group that invented the term went on to found OSI), but attached an already-existing definition (of free software) to it.