Muggles' Guide to Harry Potter/Magic/Waddiwasi

Overview
The Waddiwasi charm apparently causes violent removal of chewing gum (possibly among other things) from the place that it is currently stuck.

Extended Description
Professor Lupin uses this spell when he encounters Peeves stuffing chewing gum into a lock. He says that this is a very useful little spell, before he uses it; and it is interesting, because the effect is to fire the chewing gum up Peeves' nostril and send him away, something that many teachers had been unable to do. As to its usefulness, though, it does not seem to be used anywhere else in the books.

The incantation, the verbal part of the spell, contains the word "wad", that is to say, a small amorphous mass of material - such as chewing gum.

Analysis
As noted, despite Professor Lupin saying it is a useful little spell, it is used nowhere else in the series. This does rather call its usefulness into question, though it certainly does serve the dual purpose of clearing out the lock and getting rid of Peeves. We suspect that the author's purpose in introducing this spell is more one of enlightening us regarding the character and magical ability of Professor Lupin. The spell is clearly relatively obscure, as it serves a primary purpose (eliminating gum) that is not often needed, yet Lupin is fluent in its use, which speaks highly of his ability and memory. Additionally, he immediately sees this as a teaching opportunity, a place where he can teach the class a possibly useful spell. Many other professors at Hogwarts, we believe, would simply perform the spell, whereas Lupin makes a point of demonstrating it.