Where does “yesterday but one” come from?

yesterday but one (English) comes from English One, from Middle English on, from Old English on, from Proto-West Germanic ana, from Proto-Germanic *an, from Proto-Germanic ana, from Proto-Indo-European h₂en- — on, onto.

Ancestry of “yesterday but one”, step by step

yesterday but one traces back through 3 separate lines of ancestry.

via English One

StepLanguageWordMeaning
1EnglishOneThe number represented by the Arabic numeral 1; the numerical value equal to that cardinal number
2Middle Englishonon, in; on; Alternative form of oon
3Old Englishonon, in, at, among; from
4Proto-West Germanicanaon
5Proto-Germanic*an—
6Proto-Germanicanaon, at; on, onto; ana
7Proto-Indo-Europeanh₂en-on, onto

via English but

StepLanguageWordMeaning
1EnglishbutApart from, except (for), excluding
2Middle EnglishbutAlternative form of bote
3Old Englishbūtanwithout, outside of, except, only; outside of
4Proto-West Germanic*biūtanoutside, externally

via English yesterday

StepLanguageWordMeaning
1EnglishyesterdayThe day immediately before today; one day ago;...
2Middle EnglishyesterdayOn the preceding day; At another preceding point...
3Old Englishġiestrandæġyesterday
Every word from Proto-Indo-European h₂en- →Every word from Proto-Germanic ana →Every word from Proto-Germanic *an →
yesterday but one — Etymology, Origin & Word History | Etymology Explorer