Origins
Nine comes from Old English nigon, from Proto-Germanic *newun, from Proto-Indo-European *h₁néwn̥. It is one of the most widely attested and best reconstructed numerals in the entire Indo-European family, its cognates sitting neatly across every branch with the regular sound correspondences that make comparative reconstruction possible. Among the numbers, only three, seven, and ten are better preserved. That means we are looking at a word whose form and meaning have been continuous for at least five thousand years.
The cognate set is remarkable. Sanskrit náva, Avestan nava, Old Persian nava, Ancient Greek ennéa (ἐννέα, with a prothetic e-), Latin novem, Old Irish noí, Welsh naw, Tocharian A ñu and Tocharian B ñu, Armenian inn, Albanian nëntë, Hittite newa, Old English nigon, Old Norse níu, Gothic niun, German neun, Dutch negen, Lithuanian devynì, Latvian deviņi, Old Church Slavonic devętĭ, and Russian devyat' — every branch represented. The apparent oddity of the Balto-Slavic forms, with their initial d-, is explained as a regular assimilation under pressure from the word for ten, dekm̥, in early Balto-Slavic counting sequences. This kind of assimilation is common in the numerals of many languages: numbers are usually recited together, and adjacent forms can influence each other over generations of chanting.
What fascinates linguists is how close *h₁néwn̥ sits to *néwos meaning new (source of English new, novel, renovate, innovate, novice, and novelty). Émile Benveniste, in his 1935 work on Indo-European numerals and later in the Vocabulaire des institutions indo-européennes (1969), proposed that nine was literally the new one — the number that opened a fresh count beyond an older base-eight system, perhaps originally counted on four fingers of each hand without the thumbs. In this reading, the fingers past the eighth would have been the new fingers. The theory explains why eight (*h₁oḱtṓ, with a dual ending strongly suggesting two fours) and nine sit at a morphologically awkward seam in the number line, and why the numerals from one to seven pattern differently from those above ten. Benveniste's argument has its supporters — Oswald Szemerényi and Calvin Watkins treated the connection as plausible — but it remains disputed. The phonological match between *h₁néwn̥ and *néwos is suggestive rather than exact: they differ in vowel grade and in the presence of a final nasal. Jochem Schindler and Don Ringe have argued that the resemblance is an accident reinforced by folk etymology. The matter will probably never be settled.
Latin Roots
Latin novem carried the root through Romance into every European language. Italian nove, Spanish nueve, French neuf, Portuguese nove, Catalan nou, and Romanian nouă are the modern reflexes. The same Latin word also gave us November — originally the ninth month of the Roman calendar, when the year began in March — and nonet, a musical ensemble of nine players. The Roman calendar's failure to rename November after the calendar was renumbered in 153 BCE is a fossil: the name is wrong by two months, but it preserves the original counting. The same fossil shows in September, October, and December.
The Germanic branch went its own way with an interesting complication. Proto-Germanic *newun became Old English nigon, with a medial /g/ that developed between the nasal n and the back vowel u — a regular Germanic sound change attested across the family. Middle English lost the /g/, giving nīne (written nyne or nine). The Great Vowel Shift of the fifteenth and sixteenth centuries then raised the long ī to the modern diphthong /aɪ/, giving contemporary English /naɪn/ — the same shift that reshaped five (fīf), wine (wīn), and time (tīma). The modern English pronunciation is therefore a very recent development; Chaucer's nyne rhymed with modern mean, not with line.
Nine has gathered numerological and cultural weight in ways eight and ten have not. Three nines make a magic square of sum fifteen. The three times three grid is the logic of the Chinese nine palaces, the Persian nine heavens, and the Norse Níu Heimar (nine worlds) of the Prose Edda. In English idiom, a nine-day wonder (attested in Chaucer), a stitch in time saves nine, on cloud nine, and dressed to the nines all carry distinct senses whose origins are variously disputed — the nines in dressed to the nines is probably a corruption of to the eyne (to the eyes), but this is not certain. The ninety-nine bottles of beer song is American, dating from the early twentieth century; the nineteen-minute novena is Catholic devotional practice of medieval origin; and the cat's nine lives is a medieval English proverb already current in the sixteenth century.
Modern Legacy
November was not the only Roman institution built on nine. The novendiāle was the nine-day mourning period after a death, observed throughout the Republican and Imperial periods. Greek religion had the Ennea Hodoi (Nine Ways), an early name of Amphipolis, and the nine Muses, whose number was fixed by Hesiod in the Theogony (c. 700 BCE). The Old Norse cosmology of nine worlds, preserved in the Poetic Edda, is structurally similar. Whether these independent nine-centred schemes reflect a shared Indo-European inheritance or parallel invention is the kind of question Indo-European mythology has debated inconclusively for over a century.