Green’s Dictionary of Slang

a dark shirt, made of black or navy twill, that does not show dirt but note cit. 1918.

1918		H. Simon ‘Prison Dict.’ in AS VIII:3 (1933) 32/2: THOUSAND-MILE SHIRT. 1. A loud one, so bright-colored that it could be seen from a distance. 2. A heavy one, that would last long.
1925		J. Tully Beggars of Life 93: The usual shirt worn by tramps is one made of black satin, and is called a ‘thousand-mile shirt.’.
1935		A.J. Pollock Und. Speaks n.p.: One thousand miler, a shirt of dark colour.
1936		H. Corey Farewell, Mr Gangster! 278: One thousand miles – a dark or black shirt.
1938		L. Beebe High Iron 224: Thousand Miler: Dark blue shirt affected by railroad workers which doesn’t show spoiling.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 221/2: Thousand-mile shirt. (Hobo) A shirt of very heavy serviceable fabric; a bright-colored shirt.
1991		O.D. Brooks Legs 9: You can pawn them in the first hock shop you come to and buy yourself a pair of jeans and a thousand-mile shirt. A thousand-mile shirt is a polka dot like the rails wear. You must have noticed nearly all the brakemen and conductors wear polka-dot shirts.