A Hummer is an enormous vehicle that was designed to go off-road, although they are mostly driven by highly-paid executives (see yuppie) who need to drive a big vehicle to compensate for something smaller. Sometimes purchased by people who like to go off road, or by mothers who have a lot of kids and need something big to bring them around in, but mainly purchased by yuppies who need something to say that they make more money than you do. Hummer drivers are generally assholes who will cut you off given the chance, and won't hesitate to flip you off should you cut them off back. Hummers rarely see any dirt, except when their owners park them in the yard of their 3-story, 5-bedroom house.
Person A: "I just got a promotion, with a double salary! I'm going to use the extra money to buy a Hummer."
Person B: *dropkicks Person A*