Green’s Dictionary of Slang

1. a back that has been scarred by a judicial flogging. [the colour of the blood and subseq. the scars].

1847		[A. Harris] (con. 1820s) Settlers & Convicts 334: This poor fellow had been fifteen years in bondage, and [...] had never passed a year of that time without (let me be permitted to use a phrase not thought by any means too expressive to be regularly used at the tables of gentlemen in the colony)—without his master ‘making him a present of a red shirt’ (a scarified back).
1945		Baker Aus. Lang. 44: Red shirt [...] a back scarified by flogging.
1986		(con. 1830s) R. Hughes Fatal Shore 429: The convict [...] would show his shapes [strip for punishment] with disdain, and after the domino [last lash] he would spit at the feet of the man who gave him his red shirt.
1991		(con. 1830s) D. Neal Rule of Law in a Penal Colony 136: Argot like ‘Giving a man a red shirt’, thirty-second spaces between lashes and the descriptions of blood and flesh thrown around by the lash.

2. (US) a troublemaker; latterly mainly prison use, a recalcitrant, tough prisoner [in mid-20C US prisons’ known inmate troublemakers were issued red shirts; they thus became an easy target during a riot].

1845		N.Y. Herald 1 Mar. 1/5: An old man, named Edward Murray, [...] was placed at the bar on a charge of rioting in the street, like a juvenile red shirt.
1927		‘Jargon of the Und.’ in DN V 460: Red shirt, A refractory prisoner.
1931		Irwin Amer. Tramp and Und. Sl. 157: Red Shirt.–A refractory prisoner or one known to be ‘bad.’.
1949		Monteleone Criminal Sl. (rev. edn).
1967		M. Braly On the Yard (2002) 299: I’ve seen a lot of you red shirts come and go. Don’t none of you last long. You think you’re running everything on the yard, but you’re just a big target for anyone [...] who sees you.