I cheated. My wife remembered Stephen Jay Gould's name, but nothing about the term -- though I'd have found his name easily enough on Google.
A Google search on him suggested Wikipedia. Wikipedia had the term:
Stephen Jay Gould - Wikipedia, the free encyclopedia:Of course it's easy to then research spandrel. This must all seem mundane to young-uns, but it's still miraculous to me.
...use of the architectural word 'spandrel' in an evolutionary context, using it to mean a feature of an organism that exists as a necessary consequence of other features and not built piece by piece by natural selection...