While I'm not sure where the phrase itself came about, John Locke back in the 1600s endorsed copyright in "Liberty of the Press" as the "property" of the author

While I'm not sure where the phrase itself came about, John Locke back in the 1600s endorsed copyright in "Liberty of the Press" as the "property" of the authors and proposed an amendment to British Parliament in 1695 to "secure the author's property in his copy, or his to whom he has transferred it." (See "Lockean Property Theory and Intellectual Property" in http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1457848 http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1457848) This implies that the idea of intangible property has existed since at least the 1600s and was supported by one of the most important property theorists of history. I don't think you can dismiss it so easily as a simple "false analogy" or that calling patents and copyrights "property" is some sort of modern rhetorical invention.