Etymology
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nine (adj., n.)

"the cardinal number one more than eight or one less than ten; the number which is one more than eight;" Middle English nīn, from Old English nigen, from Proto-Germanic *newun (source also of Old Saxon nigun, Old Frisian niugun, Old Norse niu, Swedish nio, Middle Dutch neghen, Dutch negen, Old High German niun, German neun, Gothic niun "nine").

This is from PIE root *newn "nine" (source also of Sanskrit nava, Avestan nava, Greek ennea (with unetymological initial e-), Albanian nende, Latin novem (with change of -n- to -m- by analogy of septem, decem), Lithuanian devyni, Old Church Slavonic deveti (the Balto-Slavic forms by dissimilation of -n- to -d-), Old Irish noin, Welsh naw).

As "a symbol representing the number nine," late 14c. The proverbial nine lives of a cat are attested from 17c.  Nine-to-five "the average workday" is attested from 1935. Nine days (or nights) has been proverbial since mid-14c. for the time which a wonder or novelty holds attention; the expression nine days' wonder is from 1590s. The Nine "the Muses" is from c. 1600. Also see nines.

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sixty-nine (adj., n.)

"1 more than sixty-eight; the number which is one more than sixty-eight; a symbol representing this number;" see sixty + nine. In the sexual sense, 1888, as a translation of French faire soixante neuf, literally "to do 69." So called from the similarity of positions to the arrangement of the numerals.

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ninefold (adj., adv.)

"nine times repeated, nine times as great or numerous; consisting of nine parts," 1550s; see nine + -fold. Possibly a 16c. formation; the word is not attested in Middle English, but late Old English had nigonfeald.

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novenary (n.)

1570s, "an aggregate of nine," from Latin novenarius (adj.) "consisting of nine," from novem "nine" (see nine). As an adjective, "pertaining to the number nine or consisting of nine," c. 1600.

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nonagon (n.)

"plane figure with nine sides and nine angles," 1680s, a hybrid from Latin nonus "ninth" (from novem "nine;" see nine) + ending from pentagon, etc.

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novena (n.)

in Catholicism, "devotions consisting of special prayers or services on nine successive days," 1745, from Medieval Latin novena, fem. of Latin novenus "ninefold," from novem "nine" (see nine). 

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nona- 

before vowels non-, word-forming element indicating "nine," from combining form of Latin nonus "ninth," contracted from *novenos, from novem "nine" (see nine).

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decennoval (adj.)

"pertaining to the number nineteen," 1680s, from Late Latin decennovalis, from assimilated form of Latin decem "ten" (from PIE root *dekm- "ten") + novem "nine" (see nine).

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ninepins (n.)

"the game of bowls, played in an alley," 1570s, from nine + plural of pin (n.). From the number of pins to be knocked down. The game also was known as nine-pegs (1670s). Nine-holes(1570s) was a once-popular game in which players roll small balls at 9 holes made in a board or on the ground.

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nonet 

"musical composition for nine voices or instruments," 1865, from Italian nonetto, from nono "ninth," from Latin nonus "ninth," contracted from *novenos, from novem "nine" (see nine).

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