Noun
(uncountable) The illness yellow fever.
(countable, nautical) A yellow flag used by ships as a warning of disease.
(countable) The fish Caranx bartholomaei.
Source: en.wiktionary.orgA simple yellow flag (also called the "Yellow Jack") had historically been used to signal quarantine (it stands for Q among signal flags), but now indicates the opposite, as a signal of a ship free of disease that requests boarding and inspection. Source: Internet
Eliot 2006, p. 57. Al Hirschfeld drawing from Yellow Jack (Stewart, upper-left) The play was a moderate success, but times were hard. Source: Internet