E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Mouchard (French).
A spy, “qui fait comme les mouches, qui voient si bien sans en avoir l’dir.” At the close of the seventeenth century, those petits-maitres who frequented the Tuileries to see and be seen were called mouchards (fly-men). (Dictionnaire Étymologique de Menage.)
