Green’s Dictionary of Slang

1. (also graycoat) a professed Christian.

1829

A.N. Royall Mrs. Royall’s Pennsylvania I 152: [Professed Christians] have a number of names here, as in other States, ‘Grey-backs, Round-heads, &c’ [...] The sole and all-weighing cause of my partiality for the Germans, is their aversion to the gray coats, or, as they are called in Pennsylvania, blue stockings [DA].

2. an unreliable or worthless person (fig. use of grayback, a treasury note issued by the Confederate government).

1873

J. O’Connor Wanderings of a Vagabond 206: He is, however, wise enough to know when to hold his tongue, when to smile, and at whose jokes to laugh, and at a single glance can tell a ‘blood’ from a ‘grayback’.

3. (also graycoat) a Confederate soldier in the US Civil War [NB late 19C UK milit. greyback, a shirt].

1863		E.K. Wightman letter 3 May in Longacre From Antietam to Fort Fisher (1985) 132: A tempest of bullets from the pieces of our sharpshooters shattered the gray backs like chaf(f).
1864		E.K. Wightman letter 23/26 Dec. in Longacre From Antietam to Fort Fisher (1985) 224: A white flag fluttered [...] and one hundred and fifty ‘gray backs’ surrendered themselves.
1865		A.J.H. Duganne Camps and Prisons 387: The official greybacks then divide, each to a separate detachment.
1867		W.L. Goss Soldier’s Story 218: Cheatham was — a comical, jolly grayback as ever graced the Confederacy.
1883		Daily Tel. 9 Feb. 5/4: The Confederate armies [...] were known as Greybacks [F&H].
1884		G.W. Cable Dr. Sevier lvi: The gray-coats stood guard in the wavering fire-light [DA].
1894		(con. 1861–5) G.A. Sala Things I Have Seen I 237: The warriors of General Lee, who wore a light drab uniform, were known by the contumelious nickname of ‘greybacks’.
1905		(ref. to 1863) J.M. Polk Memories of the Lost Cause n.p.: It was as dark as a nigger’s pocket. I was sleepy, hungry and tired. I could feel the gray-backs moving around.
1917		J.M. Morgan Recollections of a Rebel Reefer 214: The Union soldiers craved tobacco of which the Southerners had an abundance and the ‘grayback’ longed for coffee or sugar [DA].

4. attrib. use of sense 3.

1864

J.F. Brobst letter in Brobst Well Mary, Civil War Letters 32: Well there, if that don’t beat the lousy grayback rebs.

5. (also gray one) a head or body louse; thus literary use grey-backed stranger (see cit. 1785).

1785		‘Peter Pindar’ ‘The Lousiad’ Works (1794) I 202: ‘Look, look, look, Charly! is not that a louse!’ [...] [he] with a smile the grey-back’d Stranger saw.
1855		‘When I Went Off to Prospect’ in Lingenfelter et al. Songs of the Amer. West (1968) 110: I found I was covered with body-lice; / I used unguentum once or twice, / But could not kill the grey-backs.
1862		in R.G. Carter Four Brothers in Blue (1978) 19 Aug. 82: What we first thought was the ground itch [...] proved to our uninitiated recruits to be the genuine and unmistakeable ‘greyback’.
1864		T.F. Upson diary Aug. in Winther With Sherman to the Sea (1958) 26: Someone started the story that it was full of gray backs [...] in other words body lice.
1881		Dundee Courier (Scot.) 13 May 7/6: Wonder if there’s any grey backs or jumpers in the beds?
1882		Southern Historical Society Papers x 510: These insects which in camp parlance were called graybacks, first made their appearance in the winter of 1861.
1900		Flynt & Walton Powers That Prey 192: Get it sterilized, Bony; it’s full o’ graybacks.
1911		J. Lomax Cowboy Songs 160: The fleas and graybacks worked on us, O boys, it was not slow.
1916		J.N. Hall Kitchener’s Mob 71: ‘Trench pets,’ said Shorty. Then he told me that they were not all greybacks.
1918		H. Simon ‘Prison Dict.’ in AS VIII:3 (1933) 27/2: GRAYBACK. Louse.
1920–23		G.O. Burrows in Hunter Trail Drivers of Texas (1963) I 120: Getting ‘cussed’ by the boss, scouting for ‘gray-backs’.
1922		(con. WWI) L.C. McCollum ‘Cooties’ in Lost Battalion 🌐 You can have my flock of grey ones, / For I sure have had my fill.
1926		N. Klein ‘Hobo Lingo’ in AS I:12 651: Gray backs — vermin.
1927		‘Jargon of the Und.’ in DN V 449: Grey backs, Lice.
1935		J. Conroy World to Win 61: The bo was picking the graybacks from the seams of his shirt.
1942		‘Fifty Thousand Lumberjacks’ in Lingenfelter et al. Songs of the Amer. West (1968) 507: Fifty million graybacks / Are pickin’ at their bones.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 86/2: Greybacks. A kind of vermin that infests clothing.
1965–70		in DARE.
1989		M.R. Berenbaum Ninety-Nine Gnats, Nits, and Nibblers 232: Pediculus humanus, variously known as the head louse, body louse, grayback, cootie, seam squirrel, or motorized dandruff.