Brewer's: Red Flag
(A). (i) In the Roman empire it signified war and a call to arms. (ii) Hoisted by British seamen, it indicates that no concession will be made. As a railway signal, it intimates danger, and warns the engine-driver to stop. (iii) In France, since 1791, it has been the symbol of insurrection and terrorism. (iv) It is a synonym of Radicalism and Anarchy.
“Mr. Chamberlain sticks to the red flag, and apparently believes in its ultimate success.” —Newspaper paragraph, January, 1886.