E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Battu de fol Oiseau (Etre),
or être battu de loiseau, to be utterly dismayed; to be dazed. The allusion is to bird-catching at night, when a candle or lantern is held up before the birds aroused from their sleep; the birds, being dazed, are beaten down easily with sticks.