Jelly Legs Jinx is a very brutal spell that Harry, Ron and Hermione learned in their first year at Hogwarts. However, not many people, including Lucius Malfoy and Goyle, knows that the counter curse is simply unjellify. The curse causes the victim's legs to turn to jelly rendering them immobile. This fact is strenghthened by Goyle's quote "No fair! Our Legs are jelly!!" He says this when Hermione hits him with the curse
When Harry, Ron and Hermione find Sirius Black in the castle during AVPS, Hermione hits him with the Jelly Legs Jinx but Sirius knows the counter curse so he is considered by Hermione to "be too much for them"