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):
sixties n 1: the decade from 1960 to 1969 [syn: sixties, 1960s] 2: the time of life between 60 and 70 [syn: sixties, mid- sixties]