The Collaborative International Dictionary of English v.0.48:

Forties \For"ties\, n. pl.
   See Forty.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Forty \For"ty\, n.; pl. Forties (-t[i^]z).
   1. The sum of four tens; forty units or objects.
      [1913 Webster]

   2. A symbol expressing forty units; as, 40, or xl.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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