To tremble with fear. This simile occurs in several very early French fables (thirteenth century) and was amplified by Chaucer in the fourteenth century to quake like an aspen leaf (Troilus and Criseyde, Canterbury Tales, and elsewhere). It was repeated by numerous writers over the centuries, from Shakespeare to A. A. Milne. There is good reason for the comparison to aspens in particular. The aspens, along with poplars, have flattened leaf stalks that cause their pendulous leaves to quiver in the slightest breeze.