An outdated representation of the
average American male. In the 21st century, the average American man is more likely to drink lattes, play a musical instrument, and listen to indie rock than
slobber around with a
beer belly. However, people keep using the term since they want to feel above average and so use a below average example to represent what's average.
Hey, let's hang at Starbucks and listen to Green Day while laughing at Joe Sixpack who walks around with a
beer belly, drinking
Miller Genuine Draft and listening to
Guns N Roses.
(The irony being that Green Day and Starbucks are far more popular and mainstream in the 21st century than Guns N Roses and Miller beer).