My student said, "오오… 단어 좆같에" [ohh, dan-eo joj-kat-e]. This is bad Korean cussing – literally, it means "oh, vocabulary, like a dick," but the pragmatics (the elocutionary weight of it, so to speak) might be something like "oh, vocabulary is a motherfucker."
I often understand when my students are cussing in Korean. Most of the time, unless they're insulting me or one of their peers directly, I ignore it. This is in line with the way most Korean teachers seem to handle such things, in my experience. But he'd said it right in front of me. Rather than try to call him on it, or scold him, I tried a different strategy, this time. I repeated it, exactly, right back at him.
He laughed, and one of the girls in the class put her hand to mouth, scandalized. Then he said, "Oh, Teacher, Don't Say That!"
I laughed. "But you said it."
"Oh, I know. I'm sorry, teacher."
It actually resolved really well, in my opinion. I'll have to remember this in the future.