Densaugeo

Densaugeo is a hex which causes the vicim's teeth to enlarge grotesquely at "an alarming rate".

Draco Malfoy tried to use this hex on Harry Potter in 1994, but it struck Hermione Granger instead. She was able to have nurse Poppy Pomfrey restore them to their natural state, and even managed to improve upon them.

Etymology
From Latin dens, "tooth", and augeo, "I increase" or "I enlarge".