Green’s Dictionary of Slang

a suit of clothes (jacket, waistcoat, trousers) all the same colour.

1796		Grose Classical Dict. of the Vulgar Tongue (3rd edn) n.p.: A suit of ditto; coat, waistcoat, and breeches, all of one colour.
1811		Lex. Balatronicum.
1855		Delhi Sketch Bk 1 Nov. 132/2: [cartoon caption] Mr Pimples [...] gets a new rig out [...] the latest thing in Dittos!
1863		T. Taylor Ticket-Of-Leave Man Act II: In a suit of grey dittoes, a-stepping out with another chap.
1879		‘’Arry on the River’ in Punch 9 Aug. 57/1: You’d a thought me a Turk in check dittos, and looked on your ’Arry with pride.
1880		H. Smart Social Sinners I 201: A slight, dark man, of middle height, clad in an ordinary suit of dittoes, entered the room.
1881		Herald (Melbourne) 24 Sept. 2/7: They Say [...] ‘Only two ten for a suit of superfine dittoes’.
1890		‘Rolf Boldrewood’ Colonial Reformer I 11: The labourers wore the same shoddy suits [...] the shabby-genteel people the same threadbare ditto.
1894		G.A. Sala London Up to Date 62: I should respectfully advise him [...] not to be in the habit of perambulating Pall Mall in a suit of dittoes and a pot hat.