(The). Indicative of distress. When the face is pale the “flag is down.” Alluding to the ancient custom of taking down the flag of theatres during Lent, when the theatres were closed.
“'Tis Lent in your cheeks, the flag's down.” -
Dodsley's Old Plays (vol v.p. 314, article, “Mad World.”)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894