Source : Webster's Revised Unabridged Dictionary (1913)

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[=i]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.

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®

forty
     adj : being ten more than thirty [syn: {40}, {xl}, {twoscore}]

forty
     n : the cardinal number that is the product of ten and four
         [syn: {40}, {XL}]