The Jelly-Legs Jinx or Jelly-Legs Curse is a jinx that collapses the target's legs, making them unable to stand. Its incantation is Locomotor Wibbly. It was first alluded to in Harry Potter and the Philosopher's Stone, though it was explicitly mentioned until Harry Potter and the Goblet of Fire.
This spell is included in the book Curses and Counter-Curses.
This spell was first alluded to in the first book, where the description of the book Curses and Counter-Curses mentions "Jelly-Legs" as one of the ways its reader could take revenge. It was only explicitly mentioned in the fourth book, on two different moments. First, it was used by Hermione against Harry while practising for the third task. It was also used on Malfoy, Crabbe and Goyle on the way back to London on the Hogwart's Express.