LUKE: That’s ‘Hello, nice to meet you’ in slacker.
“Slacker” is a term for someone who habitually avoids work or effort (Jess actually works at the diner before and after school every day, so can hardly be considered a literal slacker).
The phrase gained a renewed popularity following its use in the 1985 film Back to the Future, when Marty McFly and his father are referred to as slackers (and a group of teen delinquents in Back to the Future II, 1989). The 1990 comedy film Slacker, directed by Richard Linklater, about a group of twenty-something bohemians and misfits, gave it wider circulation. The 1996 comedy Clerks, directed by Kevin Smith, is regarded as the ultimate slacker cult film.
Subsequently, during the 1990s it became widely used to refer to a subculture of apathetic youth who were uninterested in political or social causes, a stereotype of Generation X. It often has connotations of an educated underachiever, or someone who is aimless in life, sometimes for philosophical or nonconformist reasons. This seems to be what Luke has in mind.