The Collaborative International Dictionary of English v.0.48:

Three-score \Three"-score`\, a.
   Thrice twenty; sixty.
   [1913 Webster]

WordNet (r) 3.0 (2006):

threescore
    adj 1: being ten more than fifty [syn: sixty, 60, lx,
           threescore]
    n 1: a set with 3 times 20 members