"1 more than forty-nine, twice twenty-five; the number which is one more than forty-nine; a symbol representing this number;" Old English fiftig "fifty; a set of fifty," from fif "five" (from PIE root *penkwe- "five") + -tig "group of ten" (see -ty (1)). Compare Old Frisian fiftich, Old Norse fimm tigir, Dutch vijftig, Old High German fimfzug, German fünfzig, Gothic fimf tigjus. U.S. colloquial fifty-fifty "in an even division" is from 1908.
<a href="https://www.etymonline.com/word/fifties">Etymology of fifties by etymonline</a>
Harper, D. (n.d.). Etymology of fifties. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/fifties