The day just behind today
In current English, yesterday is the calendar day immediately before this one. It is a noun when the day itself is the subject, an adverb when it marks when something happened, and sometimes an adjective in loose combinations such as yesterday morning. Figurative uses treat it as a name for the recent past, often with a hint that the thing in question is already out of date. Informal speech goes further still: needing a task done yesterday means needing it at once, as if the deadline had already passed. The calendar sense is the oldest of these; the others are later extensions of the same word.
Inherited, not borrowed
The origin of the word yesterday is native. It did not come in with Norman French, and it is not a later Latin learned form. Old English already used ġiestrandæġ and the two-word phrase geostran dæg: an adverb geostran (also spelled ġiestran, ġister, ġestor, ġeostran) plus dæg, the ordinary word for day. Manuscripts show several shapes of the compound, including ġyrstandæġ. The Oxford English Dictionary places first English use in the Old English period, before 1150. The adverb on its own is older still; the pairing with day is clear by late Old English. Middle English then wrote yesterdai, yisterday, ȝesterdai, and a scatter of other spellings before the modern form settled.
That second element is worth noticing because most of English’s closest relatives do without it. German says gestern and Dutch gisteren: the old time-stem alone. English kept the full compound, as West Frisian did in justerdei and Swedish in gårdag. Gothic built a similar two-part word, gistradagis, but pointed it at the following day rather than the previous one. English is therefore slightly unusual inside West Germanic for still saying the day twice, once in the inherited stem and once in the transparent word day.
The yester- half
The first half is the historically heavier piece. It goes back to Proto-West Germanic *gesteran and to Proto-Germanic *gestra- or *gesteraz. In early Germanic that stem did not yet mean only the previous day. Comparative evidence points to a sense closer to the other day, counted from today, either backward or forward. West Germanic, including Old English, locked the meaning onto the day already gone. North and East Germanic did not always follow. Old Norse gær could mean yesterday or tomorrow. Gothic, as above, used the compound for tomorrow.
Outside Germanic the family is old and wide. Latin heri and the related adjective hesternus, Ancient Greek khthés, Sanskrit hyás, Old Irish indhe, and Welsh doe all mean yesterday or something very near it. French hier continues the Latin adverb. Those matches are why etymologists treat the English form as inherited Indo-European vocabulary, not a medieval coinage.
The exact ancestral shape is disputed, and it should be left disputed. One reconstruction, associated with Calvert Watkins, is a root *dhgh(y)es- with the sense yesterday. Other handbooks write *dʰǵʰyés. German etymological work lists several related Indo-European forms and says a single starting point is uncertain. A further scholarly proposal analyzes the word as an old phrase meaning at that day, built from a demonstrative particle plus a word for day, with later simplification of a difficult consonant cluster. None of these accounts is folk etymology; they are competing technical guesses. What does not depend on any one reconstruction is the Germanic compound that English inherited and the cluster of cognates that show a very old word for a neighboring day.
The morphology on the English surface is simple once that history is granted. Yester- is a time prefix meaning the one before the present, and day is still the ordinary noun. The prefix later formed a small set of companions, some of them now literary or rare: yesternight, yestereve, yestermorn, yesteryear. The first element has nothing to do with yeast; the resemblance is accidental.
From a date to a mood
For more than a thousand years the core meaning has been stable: the day before this one. What changed is how far the word could travel from a single date. By the late fourteenth century it could also mean a short time ago, not only the previous twenty-four hours. The plural yesterdays, uncommon in everyday speech, became a poetic way to name the recent past. The modern hurry-up idiom is much later. Wanting something yesterday, in the sense of immediately, is recorded from 1974. It keeps the old backward direction from now and simply treats the past as a deadline already missed.
English also kept a contrast that the old two-way Germanic stem makes easy to miss. Tomorrow is not built from the same material. The split between yesterday and tomorrow in English is a later alignment, not a pair of twins from one root. The old stem chose a direction; another word took the other.
The Gothic reminder
The sharpest leftover in this history is directional. The same Germanic material became yesterday in Old English and tomorrow in Gothic, while Old Norse could still read it both ways. That is the clearest sign that the English word was not always a one-way arrow into the past. It began as a way of pointing to the other day, and only later did English decide, firmly, that the other day was the one already gone.