ninth (adj., n.)
"next in order or rank after the eighth; being one of nine equal parts into which a whole is regarded as divided;" c. 1300, nynþe, an alteration or replacement (by influence of nine) of nigonðe, from Old English nigoða, nigend. As a noun, "ordinal numeral corresponding to nine," late 13c. As the name of a musical interval, 1590s. Related: Ninthly.