The Collaborative International Dictionary of English v.0.48:

Bock beer \Bock" beer`\ [G. bockbier; bock a buck + bier beer;
   -- said to be so named from its tendency to cause the drinker
   to caper like a goat.]
   A strong beer, originally made in Bavaria. [Also written
   buck beer.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

bock beer
    n 1: a very strong lager traditionally brewed in the fall and
         aged through the winter for consumption in the spring [syn:
         bock, bock beer]