Excerpt from a little email banter with a Gentleman of Wit. I was explaining the origins of the phrase "raining cats and dogs" as I understood it, and used the word "etymology" in reference to the history of the phrase.
He wrote back: Can expressions have etymologies? Or is that just word origins?
Etymology is, in fact, the history of words. Singular words. But the term exists because words are elastic and mutable, and thus frequently require etymological study.
It stands to reason, therefore, that the word "etymology" itself should not be immune to such mutations, and thus I stretch its meaning to encompass turns of phrase as well, for if "etymology" won't budge an inch, then I should call it the greatest of hypocrites!