Symbolism, Imagery, Allegory
When Amory is going to high school, he quickly realizes that there's a certain type of social being who (in his mind) needs to be categorized. So he and his buddy stay up one night listing all the characteristics of a type of person known as a "slicker." The first thing Amory decides about this person is that:
The slicker was good-looking or clean-looking; he had brains, social brains, that is, and he used all means on the broad path of honesty to get ahead, be popular, admired, and never in trouble. (1.1.275)
In other words, the slicker is a goody-two-shoes, someone who always does as he's told and uses every advantage he can to rise up the social ladder. Amory hopes that he'll never be this type of person—although, truth be told, there are lots of times he feels envious toward the slicker's worldly success and prestige.
Amory's attitude toward slickers is indicative of his attitude toward, well, everything. He wants life to be easy: he wants to be popular, admired, and never in trouble. But he doesn't want to have to work to attain the attributes that make life easy, the slicker-ish qualities of "us(ing) all means on the broad path of honesty to get ahead." Quite the conundrum.
Time and again, Amory's attraction/repulsion toward what we'll term "slickerism" (say that five times fast) gets him in a sticky situation. For example, he wants to get the girl (Rosalind) but hasn't worked up the necessary social attributes—like money, status, and prestige—necessary to keep her. C'mon, this dude went to Princeton. If he wanted to tick the boxes on the path to conventional success, he totally could have. But nope: he just wants to get the reward without achieving the success.
Amory's not just a pill, though: he's a philosophizing pill. Slickerism is gross, to be sure. And especially after the carnage of WWI, slickerism seems insane-o. Why would you exert so much effort into being good and "clean looking" when all life seemed to lob at young men in the 1910s was gory violence and quashed dreams? Surely after fighting in the Great War, these dudes were owed a little happiness?