This calendar day, and this age
English uses today for the date of speaking and, just as often, for the age the speaker inhabits. The adverb still answers a simple when-question. The noun can mean this twenty-four hours or the present stretch of history. Dictionaries also allow an adjective for whatever belongs to the current moment. All of those jobs grew out of one short native phrase.
The origin of the word today is not a borrowing from French or Latin. It is an Old English prepositional phrase that fused in place and then took on extra work.
Built at home before the Conquest
Speakers already had the form in the Old English period, which standard references place before 1150, and some dictionaries push the written history back before 900. The usual shapes were tōdæġ, tō dæge, and tōdæge. Read as a gloss, they mean on this day. A late-tenth-century homily by Ælfric already uses the fused adverb for the day at hand.
The first piece is tō, a preposition that could mean at or on as well as toward. The second is dæge, the dative of dæg, the ordinary noun for day. After that preposition English needed the dative, so the whole expression began as grammar, not as a coined label. Middle English kept the same parts, often written as to dai. The Oxford English Dictionary still analyses the modern word as an English compound of the preposition to and the noun day.
Nothing in that path is a loan. The language simply stopped hearing the phrase as two free words.
The two halves, and where they stop being certain
The to inside the compound is not the later infinitive marker. It is the older locative and directional preposition. Further back it is reconstructed to a Proto-Indo-European demonstrative stem that also fed toward-words in other branches. In this particular fusion it does one job: it locates an event on the day being named.
Day is inherited Germanic. Old English dæg comes from Proto-Germanic *dagaz, the same stock as German Tag, Dutch dag, and Gothic dags. The deeper Indo-European story is unsettled, and the sources do not pretend otherwise. One proposal ties the Germanic noun to a root meaning to burn, as if a day were the hours of heat and light. Another looks toward Indo-Iranian words for day, though the initial consonant in Germanic is then hard to explain. A third reconstruction posits a separate root for day itself. What the handbooks agree on is a negative fact: English day is not related to Latin dies, despite the shared sense. For this article that dispute sits at the far end of the chain. The immediate English pieces, a preposition plus a dative of day, are not in doubt.
When the adverb learned new work
Old English used the form for on the current day and, at first, for little else. Neighboring ideas had their own phrasing. This day as a noun was a different construction. The sense we now call nowadays was carried by a phrase meaning on these days. The later English word absorbed both of those jobs.
Around 1300 the adverb could point to modern times rather than to a single date on the calendar. By the 1530s it was also a noun: this day, or any day treated as present. That stretch is ordinary for time words. A label for the moment of speaking widens until it can name the era the speaker lives in. The adjective use is a still later convenience built on the same widening.
Middle English also put the phrase to work in counting. To day seven night meant a week from the present day. Today third day meant three days from now. Those formulas dropped out of ordinary prose, but they show the old preposition still doing temporal arithmetic long after the dative ending had faded from everyday speech.
The same pattern built tonight and tomorrow. Dialect records later show to-year for this year and to-morn for the next morning. Scots kept a close relative in theday. Speakers could still see the join even after the case ending was gone.
How spelling lagged behind speech
Writing took centuries to treat the phrase as one word. Until the sixteenth century the usual spelling was two separate words. After that, hyphenated to-day became the educated default. Samuel Johnson listed the item under to, as to day, which kept the parts visible on the page. Nineteenth-century dictionaries preferred the hyphen. The solid form that now looks inevitable is largely a twentieth-century settlement. Some writers kept the hyphen well into the middle of that century, in step with to-morrow and to-night, before house style dropped it. The change was orthographic. It did not mark a new pronunciation or a new sense.
A family of strategies, not one inherited word
Other Germanic languages built the same idea with different glue. Dutch vandaag and related Low German forms are from-day. Danish and Swedish use i dag, in day. Saterland Frisian has däälich. German heute looks like a translation equivalent, but it is a different fusion: historically on this day, with a demonstrative first element rather than with to. The family resemblance is the method. Glue a small locator onto the word for day. English simply chose to.
That German mismatch is the useful souvenir. English today still contains a preposition and a dative of day. The hyphen that once advertised the join is gone, but the grammar that made the word is not.