"smoked herring" early 15c. (they turn red when cured), as opposed to white herring "fresh herring." Supposedly used by fugitives to put bloodhounds off their scent (1680s), hence metaphoric sense (1864) of "something used to divert attention from the basic issue;" earlier it simply meant "a false lead":
Though I have not the honour of being one of those sagacious country gentlemen, who have so long vociferated for the American war, who have so long run on the red-herring scent of American taxation before they found out there was no game on foot; (etc.) [Parliamentary speech dated March 20, 1782, reprinted in "Beauties of the British Senate," London, 1786]
1816, in the figurative sense of "icy reception, studied neglect or indifference," first in Sir Walter Scott, probably originally a literal figure (see cold (adj.)), but commonly used with a punning reference to "cold shoulder of mutton," considered a poor man's dish and thus, perhaps, something one would set out for an unwanted guest with deliberate intention to convey displeasure.
How often have we admired the poor knight, who, to avoid the snares of bribery and dependence, was found making a second dinner from a cold shoulder of mutton, above the most affluent courtier, who had sold himself to others for a splendid pension! ["No Fiction," 1820]
Originally with to show, later to give. As a verb from 1845; related: cold-shouldered. Also compare cold roast, old slang for "something insignificant." Cold pig was a 19c. term for throwing cold water on a sleeping person to wake him or her.
The new and exquisitely beautiful theatre of Drury-lane has the peculiar contrivance of an iron-curtain to secure the audience from all danger, in case of fire on the stage. Miss Farren, in the occasional epilogue, delivered on opening this new theatre, pleasantly informs the spectators that, should flames burst out in the part appropriated to the representation, they may comfort themselves with thinking that nothing can be burnt but the scenery and the actors. [The Monthly Review, June 1794]
From 1819 in the figurative sense "impenetrable barrier." In reference to the Soviet occupation of Eastern Europe, famously coined by Winston Churchill March 5, 1946, in speech at Westminster College, Fulton, Missouri, but it had been used earlier in this context (for example by U.S. bureaucrat Allen W. Dulles at a meeting of the Council on Foreign Relations, Dec. 3, 1945). The phrase had been used in the sense of "barrier at the edge of the sphere of influence of the Soviet Union" from 1920. During World War II, Goebbels used it in German (ein eiserner Vorhang) in the same sense. But its popular use in the U.S. dates from Churchill's speech.
The American folk custom of wearing or displaying a yellow ribbon to signify solidarity with loved ones or fellow citizens at war originated during the U.S. embassy hostage crisis in Iran in 1979. It does not have a connection to the American Civil War, beyond the use of the old British folk song "Round Her Neck She Wore A Yellow Ribbon" in the John Wayne movie of the same name, with a Civil War setting, released in 1949.
The story of a ribbon tied to a tree as a signal to a convict returning home that his loved ones have forgiven him is attested from 1959, but the ribbon in that case was white.
The ribbon color seems to have changed to yellow first in a version retold by newspaper columnist Pete Hamill in 1971. The story was dramatized in June 1972 on ABC-TV (James Earl Jones played the ex-con). Later that year, Irwin Levine and L. Russell Brown copyrighted the song "Tie a Yellow Ribbon Round the Ole Oak Tree," which became a pop hit in early 1973 and sparked a lawsuit by Hamill, later dropped.
In 1975, the wife of a Watergate conspirator put out yellow ribbons when her husband was released from jail, and news coverage of that was noted and remembered by Penne Laingen, whose husband was U.S. ambassador to Iran in 1979 and one of the Iran hostages taken in the embassy on Nov. 4. Her yellow ribbon in his honor was written up in the Dec. 10, 1979, Washington Post.
When the hostage families organized as the Family Liaison Action Group (FLAG), they took the yellow ribbon as their symbol. The ribbons revived in the 1991 Gulf War and again during the 2000s wars in Afghanistan and Iraq.