The Collaborative International Dictionary of English v.0.48:

Sixty \Six"ty\, a. [AS. siextig; akin to G. sechzig, Goth.
   sa['i]hs tigjus. See Six, Ten, and cf. Sixteen.]
   Six times ten; fifty-nine and one more; threescore.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sixty \Six"ty\, n.; pl. Sixties.
   1. The sum of six times ten; sixty units or objects.
      [1913 Webster]

   2. A symbol representing sixty units, as 60, lx., or LX.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sixty
    adj 1: being ten more than fifty [syn: sixty, 60, lx,
           threescore]
    n 1: the cardinal number that is the product of ten and six
         [syn: sixty, 60, LX]