The flag displayed from lazarettos, naval hospitals, and vessels in quarantine. (See Union Jack.)
(The). The yellow fever.
“Raymond and all his family died of yellow fever, and Fernando ... had passed a few weeks recovering from a touch of yellow Jack.” —A. C. Gunter: Baron Montez, book iv. chap. x.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894