The Collaborative International Dictionary of English v.0.48:

Fifty \Fif"ty\, a. [AS. f[imac]ftig; akin to OHG. finfzug,
   fimfzuc, G. f["u]nfzig, funfzig, Goth. fimftigjus. See
   Five, and Ten, and cf. Fifteen.]
   Five times ten; as, fifty men.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fifty \Fif"ty\, n.; pl. Fifties.
   1. The sum of five tens; fifty units or objects.
      [1913 Webster]

   2. A symbol representing fifty units, as 50, or l.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fifty
    adj 1: being ten more than forty [syn: fifty, 50, l]
    n 1: the cardinal number that is the product of ten and five
         [syn: fifty, 50, L]
    2: a United States bill worth 50 dollars [syn: fifty dollar
       bill, fifty]