Green’s Dictionary of Slang

1. a mistress, a whore.

1700		T. Brown Amusements Serious and Comical in Works (1744) III 140: Why how now, Madame Rain-bow [...] what so young a wench, and so notorious a strumpet, to have two soldiers at one time to relieve your concupiscence.
1821		Egan Life in London (1869) 193: The Pink of the Ton and his ‘Rainbow’.

2. a large, discoloured bruise, gained through boxing.

1811		Sporting Mag. XXXVII. 100: A violent blow on the forehead, by which he picked up a handsome rainbow.
1846		Bell’s Life in Sydney 27 June 3/1: Julia [...] looked as meek as two rainbow eyes and a smashed muzzle would permit.
1853		Eddowes’s Jrnl 14 Dec. 2/4: One gigantic culprit has been beaten into a very rainbow of bruises, probably in some savage encounter with the police.
1935		M. Fulcher ‘Believe Me’ in Afro-American (Baltimore, MD) 23 Feb. 12/1: They inhale a load of my bandaged pan and this rainbow effect on the eye and let olut shrieks.

3. a pattern book.

1821		Egan Life in London (1869) II, i: Now, Dicky, out with your rainbow. Here are the patterns, gentlemen, the very latest fashions.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.

4. a footman, abbr. (late 18C) knight of the rainbow, a footman in livery.

1821		Egan Life in London (1869) 347: [note] A knight of the rainbow; a cant phrase for a footman in livery, in allusion to the various colours of his jacket.
1830		Lytton Paul Clifford II 44: A lean footman in livery, – such a livery, ye gods! – scarlet, blue, yellow, and green, a rainbow ill made.
1882		Breckenridge News (Cloveport, KY) 23 Aug. 3/3: One day I met a ‘rainbow’ that was ‘sassy’ and called him a ‘soldier’.

5. a golden guinea [its shape; Partridge suggests link to rhino n.¹ (1)].

1937–84

Partridge DSUE (1984) 957/1: from ca. 1850; ob.

6. a young man about town.

1821		W.T. Moncrieff Tom and Jerry I iv: Barber’s clerks, costard-mongers – swell coves, and rainbows, all jostle one another.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
1895		C. Crowe Aus. Sl. Dict. 64: Rainbow, a gaudily-dressed person.

7. (US black) ? a heavily-pregnant stomach [the pronounced curve resembled that of a rainbow].

1934

C.S. Johnson Shadow of the Plantation 64: ‘Girls what ain’t married don’t never ought to get deyselves no rainbow’.

8. (W.I.) a tall, thin person.

1958

cited in Cassidy & LePage Dict. Jam. Eng. (1980).

9. in drug uses.

10. (US black) one who dresses in gaudy bad taste.

1980

E. Folb Runnin’ Down Some Lines 42: Rainbow, color guard, christmas tree, Santa Claus [...] refer specifically to a person who wears too many colors.