ninetieth 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.

word-forming element making ordinal numbers (fourth, tenth, etc.), Old English -ða, from Proto-Germanic *-tha- (cognates: Gothic -da, -ta, Old High German -do, -to, Old Norse -di, -ti), from PIE *-to-, also *-eto-, *-oto-, suffix forming adjectives "marking the accomplishment of the notion of the base" [Watkins].

Cognate with Sanskrit thah, Greek -tos, Latin -tus; Sanskrit ta-, Lithuanian and Old Church Slavonic to, Greek to "the," Latin talis "such;" Greek tēlikos "so old, of such an age," Old Church Slavonic toli "so, to such a degree," toliku "so much," Russian toliko "only;" also see -ed.