You are correct, fossil fuel perhaps being a poor example on my part. Although my point stands that when a word is used interchangeably, in this case with ethnicity, it becomes the commonly accepted term socially.

That said, the word fossil for use to refer to geological remains of plants and animals may indeed have predated the use of the term fossil fuel. I wonder how reliable Wikipedia is as a source of historical references when it is only as concise as the information added to it publicly. Both terms would have originated in a similar time period.