There is an interesting tension/distinction between "slacker" and "nihilist" in The Big Lebowski. While the primary "conflict" in the film takes place between the square, conservative, ex-vet Big Lebowski and the burnt-out, draft-dodging Dude, there is also a sub-conflict between nihilists and slackers. Both Walter (John Goodman) and The Dude seem to have disdain for nihilists, amusingly represented by three German techno musicians as well as by Bunny's pornstar boyfriend.
The film is filled with overt references to nihilism, which can be defined as:
the philosophical stance that existence is without meaning, purpose, or intrinsic value, and that there is no morality whatsoever.
And, although I don't think the term "slacker" is overtly used, the Big Lebowski himself refers to The Dude as a "bum", which we can take to mean the same thing. A quick jump to Wikipedia gives us this definition of the term slacker:
a term commonly used to refer to a person who avoids work or military service, or (primarily in North American English) an educated person who is anti-materialistic and viewed as an underachiever.
So what makes a slacker so amusing and loveable while a nihilist is an object of derision? I'd say that the fundamental difference lies solely in the way an individual perceives the philosophy and projects it onto themselves. That is, there really is no difference between a slacker and a nihilist other than that nihilists have made "caring for nothing" an ethos and point of introspection, while slackers are too lazy or too stupid to focus on the matter.
In a sense, The Dude is more nihilistic than a nihilist because he doesn't obsess over his nihilism. Theoretically, a nihilist shoudln't care about anything, even his/her own worldview. So, it seems that a nihilist is just a slacker who took his slacking a little too seriously.
Of course, the joke could be on the slackers as well. They deride and disparage the nihilists, all the while there is no morality or meaning in their own lives. The slackers are actually disgusted by a mirror image of themselves, albeit an image that takes the "philosophy" far more seriously...and makes bad German techno.