So periodically students ask me for words that aren't in the textbook. About a month ago, a boy pantomimed the motion of being punched in the nuts and asked, "What's this?"
I said, "Oh, that's a cheap shot." But then the boy started going around asking, "Cheap shot ok? Cheap shot ok?"
I decided I would try to explain to the boy that "cheap shot" meant that it wasn't a good thing to do, so I pantomimed punching someone in the back and said, "This is also a cheap shot." But that didn't work, so I said, "Cheap shot... like cheap hamburger" hoping to draw attention to the meaning of "cheap." Instead, the boys (there were several of them by now) heard "cheap shot", thought of punching each other in the crotch, and then thought "cheap hamburger"... Well, as you can imagine, it was a disaster.
Also, somewhat unrelated, my students are fond of making words of their own. Today, though, we had a real winner-- "Patrick shock", which refers to the process (and feeling?) of almost crashing into me as you too fast around a corner. Which is, in fact, an ever present danger for several students who seem to be in the habit of running without looking.