fifties, the numbers, years, degrees, or the like, from 50 through 59, as in referring to numbered streets, indicating the years of a lifetime or of a century, or degrees of temperature: She lives in the East Fifties. He's in his late fifties. It's going to be in the fifties again today.
5.
Informal.a fifty-dollar bill: He had a fifty and two tens in his wallet.
O.E. fiftig, from fif "five" + -tig "group of ten." (cf. O.N. fimm tigir, Du. vijftig, Ger. fünfzig). U.S. colloquial fifty-fifty "in an even division" is from 1913.