It is my understanding that the term “etymology” properly applies only to individual words alone. Priberam has for etimologia:
e·ti·mo·lo·gi·a
(latim etymologia, -ae, do grego etumología, -as)
substantivo feminino
- [Gramática] Parte da Gramática que trata da origem e formação das palavras.
- Origem de uma palavra. = ÉTIMO
Citation: "etimologia", in Dicionário Priberam da Língua Portuguesa [em linha], 2008-2013, http://www.priberam.pt/dlpo/etimologia [consultado em 05-09-2016].
And for étimo Priberam has:
é·ti·mo
(grego étumon, -ou, verdadeiro significado de uma palavra, de acordo com a sua origem, do grego étumos, -e, -on, verdadeiro)
substantivo masculino
- [Gramática] Vocábulo considerado como origem de outro.
- Origem de uma palavra. = ETIMOLOGIA
"étimo", in Dicionário Priberam da Língua Portuguesa [em linha], 2008-2013, http://www.priberam.pt/dlpo/%c3%a9timo [consultado em 05-09-2016].
And Wikipédia says:
Etimologia (do grego antigo ἐτυμολογία, composto de ἔτυμον "étymos" e -λογία "-logia") é a parte da gramática que trata da história ou origem das palavras e da explicação do significado de palavras através da análise dos elementos que as constituem. Por outras palavras, é o estudo da composição dos vocábulos e das regras de sua evolução histórica.
So etymology is more than just the historical origin of a term. It is an analysis of the previous forms of the word, including the evolutionary morphology, by which I mean the historical changes in the little pieces that make up the word.
Longer utterances, such as famous sayings and other idiomatic expressions, do of course have histories behind them, whether we know those or not. They may even have shifted in exact form and meaning over time.
Here’s the thing though: I don’t know that most visitors of our site are thinking of narrow distinctions like these when they ask for the etymology or origin of a word or a phrase. I doubt they are thinking “what are the previous forms of this word?” They likely just want to know how it has come to mean what it means or be written the way it is today written.
So although I in my own head distinguish the formal term etymology as applied to one single word in the language, a vocábulo, it may be asking more of our visitors than is entirely reasonable to expect them to use the term etimologia in only this very restricted way and for them to select some other, broader term when it applies to longer strings of words taken together.
However the Community here decides to handle this can be fixed up quickly and easily with the tags. Just remember that tag merges are destructive, and so if we merge everything there’s no sorting them out again later if we later decide otherwise.