campaign coat, n. — Green’s Dictionary of Slang

campaign coat n.

[orig. a milit. uniform, then in civilian tailoring a style of coat that resembled military uniform; as worn by a beggar, such a coat was supposed to present the image of an old soldier]

a tattered old coat, worn by beggars spec. to excite sympathy in passers-by.

c.1698		B.E. Dict. Canting Crew n.p.: Campaign-coat, originally only such as Soldiers wore, but afterwards a Mode in Cities.
1725		New Canting Dict. n.p.: Campaign-coat, in a Canting Sense, the ragged, tatter’d [...] Coat, worn by Beggars and Gypsies, in order to move Compassion.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].