"Hey!" Dean shouted out in his defense. "Those words are ... " He trailed off as he tried to think of something that wasn't 'hard'. If she looked at him, she would have seen the wheels turning. "not in English. Do you know how long it takes some people to learn another language? Their entire lives! I know most of it." And he didn't add the fact that he never really bothered with knowing more than that because he had always relied on his father or Sam. Which wasn't a mistake that he typically made but compared to all the things he had to do as a hunter, Latin-learning was amongst the most boring. And Dean didn't do well when he was bored. There was immediate snoring involve. Sammy was the walking encyclopedia of dork and weird. He just shot stuff and was real good at it too.
"That was good." Dean said mostly to maybe ease her ire, and because it was good. Real sharp that comment. "Real, uh, quippy." It did filter in his brain that she likely wouldn't have given a damn about his compliment but Dean was one that tended to say anything at any given situation. No matter who the other person ... or monster he was with was. Hell, like more especially, when they could kick is ass.
He was already making his way back to his car, after the comment about his approval of her quippiness, when she asked where it was, "I'm getting it! Just hold him off for a couple of seconds." That comment was likely to get him smacked. He knew that he was going to get smacked, but he jogged his car anyway, which wasn't that far away. And got the book.
He made his way back impressed that Buffy was still going.