The Collaborative International Dictionary of English v.0.48:

Billy \Bil"ly\, n.
   1. A club; esp., a policeman's club. Also called billy club
      [1913 Webster +PJC]

   2. (Wool Manuf.) A slubbing or roving machine.
      [1913 Webster]

WordNet (r) 3.0 (2006):

billy
    n 1: a short stout club used primarily by policemen [syn:
         truncheon, nightstick, baton, billy, billystick,
         billy club]
    2: male goat [syn: billy, billy goat, he-goat]