Ponyboy mentions this in the course of relating how he, Johnny and Dally met the two Soc girls, Marcia and Cherry for the first time. This brings up the whole issue of how Greasers behave towards girls in general. Ponyboy explains as follows:
When Steve's cousin from Kansas came down, Dally was decent to her and watched his swearing. We all did around nice girls who were the cousinly type. I don't know how to explain it--- we try to be nice to the girls we see once in awhile, like cousins or the girls in class; but we still watch a nice girl go by on a street corner and say all kinds of lousy stuff about her. Don't ask me why. I don't know why.
Ponyboy here observes that while Greasers treat the girls that they know with respect, they tend to act up around strange girls, saying 'lousy' things about them; and this puzzles him when he comes to think about it. It just seems to be part of the Greaser culture, a kind of showing off, a way of living up to their mean, tough, hoodlum image, and probably a means of seeking attention from an attractive girl. Dally especially revels in this kind of thing, as seen in the way that he 'talks dirty' in front of Cherry and Marcia, the two girls who are strangers and, moreover, Socs. However, as Ponyboy recalls, he didn't swear in front of the female cousin of a fellow Greaser, because he evidently regarded her as part of the Greaser family.