An underachiever or irritatingly laid-back character.
Possibly disorganised, possibly prone to procrastination.
There can be a bit more to it than mere laziness or sloppiness, a perception that the rat race isn't worth it and shop-till-you-drop a surrogate happiness of low quality, and a slow poison, too. So this kind of slacker might (or might not) be dedicated to what he or she does (paragliding, for example), but would probably not list “climbing the corporate ladder” as an ambition or “shopping” as a hobby. May be a bit of a “Generation X” thing.
Hear Beck's song “Loser” or see the video for Soundgarden's “Black Hole Sun”.
I mean, most of the girls I went to school with were crazy about buying all kinds of stuff and they would disappear in shoe shops for days, but list “shopping” as a hobby? No one would have thought of it as a damn “hobby”! And even if someone had, they still wouldn't have dared to admit they're so damn shallow. What has the world come to?
G: “Yeah, I know. Nothing to be done about that. The shits are doomed and so are we. You should see their parents. Now get off your butt and out of the car, you little slacker, we still need to get groceries.”
Tends to be sort of affectionate slang in this context.
2) Synonym for a typical member of Generation X (born 1964-1981 roughly). Anyone who smoked reefer, wore flannel or owned the Chronic, Doggystyle or any music by Sublime, the Red Hot Chili Peppers, Pearl Jam , Nirvana, Rancid , etc., in the first half of the 1990s. Anyone who liked movies directed by Quentin Tarantino.
Tends to be neutral slang, when used in this context.
3) Antonym of a politically aware member of the Baby Boom generation (born 1946-1963 roughly). Someone who thought of the Beatles, Vietnam, Woodstock or JFK being as historically relevant today as the Civil War or the War of 1812.
In this context is usually pejorative and insulting.
4) A description of a young man in the early 20th Century, who avoided military duty. An amoral, lazy coward.
Also negative in this context
2) Wow. Lots of Slackers at Lollapalooza this year again. I wish someone would shoot Ani DeFranco.
3) You fucking Slackers think that Reaganomics was cool. You probably never even heard of Kent State or the Montgomery Bus Boycott. Too bad you weren't old enough to have had to have been drafted and sent to Vietnam.
4) You yellow-bellied slackers will be court-martialed, when General Patton returns from the Battle of the Bulge.