Ninety

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Old English nigontig .

wiktionary

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From Middle English nynety, ninty, nineti, nigneti, from Old English hundniġontiġ, from Proto-Germanic *newuntēhundą(“ninety”), equivalent to nine +‎ -ty. Cognate with Scots nynty, nynety(“ninety”), Saterland Frisian njuugentich(“ninety”), West Frisian njoggentich(“ninety”), Dutch negentig(“ninety”), German Low German negentig(“ninety”), German neunzig(“ninety”), Swedish nittio(“ninety”), Norwegian Bokmål nitti(“ninety”), Norwegian Nynorsk nitti(“ninety”), Icelandic níutíu(“ninety”).

etymonline

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ninety (adj., n.)

"9 times ten; the number which is one more than eighty-nine or 10 less than one hundred; a symbol representing this number;" Middle English nīntī (late 13c.), from Old English nigontig, from nine + -tig "group of ten" (see -ty (1)). Cognate with Old Frisian niontich, Middle Dutch negentich, Dutch negentig, German neunzig, Old Norse nintigir.