Shaking like a Sumac Leaf is a saying to mean that someone's body is shaking a lot.
For example if your body is shaking because you are very cold or frightened then you could be shaking like a sumac leaf.
In the wind a Sumac Leaf or other leaf on a tree will be shaking constantly so that is where that saying comes from.
Other sayings are you're shaking like a leaf or shaking like a peach tree etc.
It just means that you're shaking uncontrollably.
A sumac leaf comes from the Sumac Tree and a Sumac Tree or Sumac is any one of about 35 species of flowering plants in the genus Rhus and related genera, in the family Anacardiaceae.
The Sumac Tree grows in subtropical and temperate regions throughout the world, especially in East Asia, Africa, and North America