fifty (n.) Look up fifty at Dictionary.com
Old English fiftig "fifty; a set of fifty," from fif "five" (see 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.