nervous as a cat on a hot tin roof

If someone is as nervous as a cat on a hot tin roof they are in a state of extreme nervous or worry, are very excitable in a negative way and are practically sitting on the edge of their seat with anxiety. Cats who are on a hot tin roof wont be able to stand still even for a second due to being worried about the heat and of falling off the roof; this is why the idiom is so fitting for someone in severe nervous or worry.