When a woman has an orgasm so intense her legs become like jelly, possiblyresulting in her falling over. Often accompanied by a Squirt
Laura: I'm telling you, sit down when you jill off in the shower.
Sarah: Why?
Laura: Because if you do it right you'll get Jelly Legs and you don't want to hit your head on the way down.
When a child goes into a state of paralysis below the waist in order to get what he/she wants.
Kid: I WANT CANDY!!
Mom: No!
Kid: I WANT CANDY!(Kid goes into jelly legs mode)
Mom: Timmy stand up! I'm not going to carry you!
Kid: WAAAAAAAHHHH!
Mom: Jesus! Here...what do you want?