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]

The Collaborative International Dictionary of English v.0.48:

Forty \For"ty\ (f[^o]r"t[y^]), a. [OE. forti, fourti, fowerti,
   AS. fe['o]wertig; fe['o]wer four + suff. -tig ten; akin to
   OS. fiwartig, fiartig, D. veertig, G. vierzig, Icel.
   fj["o]rut[imac]u, Sw. fyratio, Dan. fyrretyve, Goth.
   fidw[=o]r tigjus. See Four, and Ten, and cf. Fourteen.]
   Four times ten; thirty-nine and one more.
   [1913 Webster]

WordNet (r) 3.0 (2006):

forty
    adj 1: being ten more than thirty [syn: forty, 40, xl,
           twoscore]
    n 1: the cardinal number that is the product of ten and four
         [syn: forty, 40, XL]