Green’s Dictionary of Slang

1. (US) someone who hid themselves to avoid conscription during the American Civil War.

1865		Dly Phoenix (Columbia, SC) 1 Oct. 2/1: I am a mossy-back, sir, and I stand here to-day to represent the county of Jones, otherwise known as the free State of Jones.
1865		Burlington Wkly Hawk-eye (IA) 10 June 3/5: The 2nd class are [...] ‘mossbacks.’ They are men who from one cause or another have refused to join either side, and to avoid the conscript acts have ‘laid out’ in the woods.
1872		Schele de Vere Americanisms 283: The Mossyback […] was the man of the South, who secreted himself in a remote forest, or an inaccessible swamp, in order to escape conscription. His name was derived from the quaint fancy that he was determined to keep in hiding till ‘the moss should grow on his back’.
1890		Century Dict. 3869/3: moss-back [...] 3. In the southern United States, during the civil war, one who hid himself to avoid conscription [Slang].

2. (US, also mosshead) a diehard conservative.

1871		Eve. Star (Wash., DC) 12 Aug. 1/5: The progressionists understand that ‘the whole country has an interest in the result’ [...] while the mossbacks insist on keeping their money for shaving purposes.
1878		St. Louis Clinical Rev. I 92: A ‘bigot,’ a ‘moss-back,’ ‘an old fossil, determined to live and die by the Allopathic practice at all hazards;’ could not and would not renounce the belief and practice of a life time.
1885		Boston Journal 5 Mar. n.p.: Everyone rejoices over the passage of the bill . . . except a few intense mossbacks, who were known during the war as copperheads [R].
1893		Harper’s Mag. 87 July 305/2: When a blinded conservative is called a mossback, who is so dull as not to perceive the poetry of the word?
1902		S.E. White Blazed Trail 126: I took her to a dance one night, / A mossback gave the bidding – / Silver Jack bossed the shebang, / And Big Dan played the fiddle.
1900		Nat. Police Gaz. (NY) 22 Sept. 6/1: [headline] abbreviated costumes / Worn By Massachusetts Summer Girls / shock the mossbacks.
1904		Ade ‘The Fable of the Old Fox and the Young Fox ’ in True Bills 151: A Rolling Stone gathers no Moss and therefore will not be derided as a Moss-Back. Roll as much as possible.
1904		Goodwin’s Wkly (Salt Lake City, UT) 19 Nov. 4/1: Even old mossy-backed Democrats said: ‘That reminds us of Andrew Jackson, and the reminder is a pleasant one’.
1913		Sat. Eve. Post 15 Dec. 6: Referring to a moss-back a brakeman said: ‘Why, he’s still knitting socks for the soldiers’.
1927		(con. 1920s) S. Lewis Elmer Gantry 419: The hell-howling of moss-backs corrupt the honest liberals a lot more than the liberals lighten the backwoods minds of the fundamentalists.
1934		O. Strange Sudden 56: Purdie said there was one ol’ mosshead who would mebbe make trouble.
1948		Chicago Daily News 25 Sept. 6/1: The ‘mossbacks’ in Congress, ‘big business’ and the ‘gluttons of privilege’ are his favorite targets [DA].
1957		H.B. Allen ‘Pejorative Terms for Midwest Farmers’ in AS XXXIII:4 265: [...] mossback.
1968		W. Safire New Lang. Politics 267: MOSSBACK a reactionary; one who furiously resists progress of any kind. The word is derived from a sea creature so ancient that it has moss or seaweed growing on its back.
1978		A. Maupin Tales of the City (1984) 56: All the old mossbacks would [...] say how nice it was to know that there were still some decent, upstanding young men left.
1989		H. Rawson Dict. of Invective (1991) 257: mossback. An extreme conservative.
1994		M. Ribowsky Don’t Look Back 314: [O]nly Satchel Paige could have made Jackie Robinson line up on the side of the baseball mossbacks.

3. a recluse.

1883		C. Halleck Sportsman’s Gazeteer 700: Mossback.— A settler; a homesteader; a pioneer farmer. (Western).
1891		C. Roberts Adrift in America 249: He was followed to the very verge of the wood, and then the exhausted ‘mossback’ left him to return to the house.
1902		S.E. White Blazed Trail 66: Regular old backwoods mossback.
1924		J. Buchan Three Hostages in Buchan (1930) 852: I was becoming such a mossback that I had almost stopped reading the papers.