"1 more than sixty-nine; the number which is one more than sixty-nine; a symbol representing this number;" Old English (hund)seofontig, from seofon (see seven) + -tig (see -ty (1)). Similar formation in Old Frisian soventich, Middle Dutch seventich, Old Norse sjau tiger.
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.
<a href="https://www.etymonline.com/word/seventieth">Etymology of seventieth by etymonline</a>
Harper, D. (n.d.). Etymology of seventieth. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/seventieth