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general::
Also, shake in one's boots; quake or shake like a leaf. Tremble with fear, as in The very thought of a hurricane blowing in makes me quake in my boots. Both quake and shake here mean "tremble." These idioms were preceded by the alliterative phrase shake in one's shoes in the late 1800s. The idioms with leaf allude to trembling leaves, as in He was shaking like a leaf when the exams were handed back. A similar expression was used by Chaucer, who put it as quake like an aspen leaf, a particularly apt comparison since aspen leaves have flattened stems that cause the leaves to quiver in the gentlest breeze.
American Heritage Idioms