The night that belongs to this day
English speakers use tonight as both an adverb and a noun. As an adverb it points to the evening or the hours of darkness that follow the current day: a meeting after work, a storm after dusk, a broadcast that starts once the light has gone. As a noun it names that same stretch of time as something you can wait for, plan around, or remember in the morning. The word is so ordinary that it barely feels assembled. Its history is not a French or Latin loan. It is a native English time-phrase that took more than a thousand years to look like a single token.
Built at home
The origin of the word tonight is a compound formed inside English. In Old English the pieces were the preposition tō and the noun niht. Written as a phrase, tō niht, and sometimes already compacted as toniht, it meant the night now present or the night still to come. Historical dictionaries place first use in the Old English period, before 1150. Homiletic prose from around the year 1000 already shows the fused spelling. Later desk dictionaries point to the same source: English to plus English night, not a borrowed adverb from the Continent.
Middle English kept those parts, with spellings such as to niȝt, tonyght, and to night. By the early fourteenth century the phrase could also stand as a noun for the night after the present day, the sense still heard when someone talks about tonight as a date rather than as a modifier.
What the first half was doing
The opening element is the same to that now mainly marks direction or an infinitive. In Old English it also marked a point or span of time: at, on, in the course of. That temporal use survives in a tiny closed set. Today comes from tō dæge, tomorrow from tō morgenne, and tonight from tō niht. Each began as a prepositional phrase with a time-word in a dative-like form. Modern English no longer builds new members of the set. There is no living tomorning or tonoon, because the construction froze while to could still mean on this unit of time.
West Germanic languages share the preposition itself. Old Saxon and Old Frisian had to, Dutch has toe, German zu. Scandinavian languages generally use a different particle, the ancestor of English till. A deeper reconstruction ties to to a Proto-Indo-European demonstrative stem, the family that also yields Latin donec and Old Church Slavonic do. None of that deeper history is required to hear the English compound. The local sense at or on is enough.
The second half, and its relatives
Night is inherited Germanic. Old English niht, with West Saxon neaht and Anglian næht or neht beside it, continues Proto-Germanic nahts. Close kin include Old High German naht, Dutch nacht, German Nacht, Old Norse natt, and Gothic nahts. Beyond Germanic the same root is unusually widespread: Latin nox, Greek nyx, Lithuanian naktis, Old Church Slavonic nosti, Russian noch, Old Irish nochd, Sanskrit naktam. The reconstructed ancestral form is often given as nekwt. A further link to a verbal root meaning to be dark is sometimes offered; that step is less secure than the noun itself, and the article need not pretend otherwise.
The English vowel of night comes from the old oblique cases, genitive nihte and dative niht, rather than from the nominative. The gh in the modern spelling is a Middle English writing habit shared with words such as fight. It does not mark a separate etymological consonant here.
Other languages name this same slice of time with compact words built on that night root rather than on an English-style to plus night. Welsh henoid, Irish anocht, and Scottish Gaelic a-nochd sit in that group. German and Dutch usually take another route, pairing a word for this day with a word for night: heute Nacht, vannacht. English chose the frozen preposition instead.
A sense that fell away
Old and Middle English did not pin the compound to one clock. Middle English usage covered this very night, the night just past, the night still to come, and, as a noun, this present night. The last-night reading lasted into Early Modern English and then shrank to dialect. A speaker in the morning could still, in that older usage, refer to the darkness just ended with the same word later reserved for the darkness ahead. Standard English later split the work: last night for the past, tonight for the coming evening and night.
That older looseness fits an older calendar. In early English reckoning the day often began at sunset, so a named night could mean the darkness before the named day. Old English monanniht, Monday night, was the night leading into Monday, which later clocks would call Sunday night. Other weekday and feast-night compounds show the same shift. Tonight itself was never a weekday label, but it lived in a world where night was a unit you counted with, not only a stretch after supper.
How the spelling caught up
For centuries the two halves stayed visible on the page. They were commonly written as two words until the eighteenth century. Then a hyphen became usual, matching the period spellings of today and tomorrow. The solid modern form belongs to the early twentieth century, later than the fusion of today. Informal American shortenings such as tonite sit outside the etymology; they only respell a word that had already fused.
What remains is a fossil of a preposition that once meant on this time, attached to one of the oldest time-words in the Indo-European family. Welsh henoid is a useful reminder that English did not need a loan to name this night. It only needed to glue two native pieces and then wait for the glue to show.