be like jelly

To be or feel very unstable or weak. This phrase is often used to describe the knees or legs. My legs were like jelly when I got off that roller coaster—I had to sit down right away. I was doing OK until I saw my wife in her wedding gown at the end of the aisle—then, my heart was pounding and my knees were like jelly. Whew, it's good to be inside—my legs were like jelly after standing in that blazing heat all afternoon.

be/feel like ˈjelly

(also turn to ˈjelly) (of legs or knees) feel weak because you are nervous or frightened.: She couldn’t move — her head was swimming, her mouth was dry and her legs felt like jelly.