I have always heard the term used in referring to a single word. When browsing questions on this site, I've seen it used applied to entire phrases, and have suppressed the compulsion to edit them and replace the term with origin.
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The key phrase there is "linguistic form". Words and morphemes are linguistic forms, but so are sentences and phrases. Sentences and complex phrases are not fixed enough to be really studied in an etymological sense, but common phrases, idioms, and other fixed forms are, so I see no problem with applying the term "etymology" to those things. Furthermore, the boundaries between affix, clitic, word, and phrase are very murky, and I don't see any reasonable criterion for allowing the first three to have etymologies, but not the fourth.
Here are my two coins:
I looked up the dictionary, and etymology is defined thus:
So, etymology is used on individual words, roots, prefixes, suffixes, affixes, etc. But not on phrases, idioms, or expressions.
For phrases, idioms, or expressions, I would use:
To sum up, for me, etymology is only used for individual words and morphemes, while origin is used for phrases, idioms, and expressions.