A period of time during a school year when academic responsibilities are pushed aside in favor of non-academic pursuits. In slacker terms, an entire semester of going to the bar every night instead of studying. See also, alcoholism.
While it may happen at any point in a college, graduate, or high school career, typically a slackmester occurs in the last semester of a given school period, when students suddenly realize how pointless hard work really is.
Dave: "Tacos and then the bar later tonight?"
Adam: "Hmmmm Tuesday night, two papers due tomorrow at 8 AM, and I haven't slept since Sunday... count me in. I'm obligated as part of my slackmester.