[zenhack]

I don't have any great insight that's not in the public record, but from reading that announcement and the other article you linked, observations:

- There are a bunch of references to other folks using the term, all of which are big names I recognize as being pioneers in open source consistent with the OSI's notion of it.

- The mailing list posts enumerate a bunch of advantages that would not follow from mere availability of source code without additional rights consistent with the OSI's general notion.

The sense I get from this is that the term probably was in use already, but it pretty much meant what the OSI says it does, not just "the source is out there."