“Open source” can be legitimately used as generic term, and as such one doesn’t need any definition at all to use the term in its colloquial sense.
Imagine someone were to arbitrarily define the term “free lunch” to mean something very specific, instead of the naive interpretation of “a meal at noon that you don’t have to pay for”. Only because they’d claim and publish their definition doesn’t make it universal from then on, regardless of whether the combination of those words had been used somewhere else before.
If you think of patent law, for example, it’s not for nothing that you can’t claim a trademark on generic terms. The reasoning behind that stems from the very same problem.
I agree with you that it probably would have been better if the FOSS movement had established a more specific term for what they meant. (Like: “FOSS”, for example.) But – to put it bluntly – that’s a home-made problem on behalf of the FOSS movement, and not mine.