The Collaborative International Dictionary of English v.0.48:

beer belly \beer" bel`ly\ n.
   a protruding potbelly caused by excessive drinking of beer.

   Syn: beer gut.
        [PJC]

              In one of the less surprising revelations of the
              year, researchers at the University of North
              Carolina (UNC) and colleagues have confirmed that
              excessive consumption of beer can lead to the
              condition commonly known as beer belly. At the
              same time, however, they discovered that, beyond
              aesthetic concerns, the condition may point to
              health hazards of a more serious nature. In a
              comparison of beer drinkers and wine drinkers, the
              scientists found that beer tends to build a
              central paunch, or "potbelly", while wine drinkers
              tend to have narrower waists, even when the same
              amount of alcohol and calories is consumed by
              both.                               --The
                                                  Scientist --
                                                  December 11,
                                                  1995.