E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Coup de Pied de lAne (kick from the asss foot).
A blow given to a vanquished or fallen man; a cowardly blow; an insult offered to one who has not the power of returning or avenging it. The allusion is to the fable of the sick lion kicked by the ass. (French.)