E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The aspen leaf is said to tremble, from shame and horror, because our Lords cross was made of this wood. The fact is this: the leaf is broad, and placed on a long leaf-stalk so flexible as scarcely to be able to support it in an upright position. The upper part of the stalk, on which the play mainly depends, is flattened; and, being at right angles with the leaf, is peculiarly liable to be acted on by the least breath of air.
Aspen leaf. Metaphorically, a chattering tongue, never quiet.
Those aspen leaves of theirs never leave wagging.Sir T. More.