Source : Webster's Revised Unabridged Dictionary (1913)

Forties \For"ties\, n. pl.
   See {Forty}.

Forty \For"ty\, n.; pl. {Forties} (-t[i^]z).
   1. The sum of four tens; forty units or objects.

   2. A symbol expressing forty units; as, 40, or xl.

Source : WordNet®

forties
     n 1: the time of life between 40 and 50 [syn: {mid-forties}]
     2: the decade from 1940 to 1949 [syn: {1940s}]