scarlet, n. — Green’s Dictionary of Slang

scarlet n.

1. an upper-class ruffian [a synon. for blood n.¹ (1)].

1755

J. Shebbeare Lydia (1769) II. 437: I expected to have seen her [...] encouraging the young bloods, bucks and scarlets at a riot in Drury-lane, the most distinguishing and newest Mark of a Lady ' of Quality.

2. one who rides to hounds [the ‘pink’ coat].

1828		High Life in London 10 Feb. 8/2: Lord Stanley, who rode a beautiful clip’d grey; was now the most forward of the scarlets.
1859		Bell’s Life in Tasmania 26 July 3/5: One straight going scarlet had the ill luck to lose his horse, from the latter falling at a stiff fence.

In phrases

bit of scarlet (n.)

(gay) soldiers willing to rent themslves out for homosexual encounters .

1992

J. Gardiner Class Apart 53/2: The neighbourhoods around Guards’ barracks [...] were well known trolling grounds, and many of the pubs in the Knightsbridge area became notorious for gay men looking for ‘a bit of scarlet’ (nineteenth-century slang for a soldier) [Simes:DLSS].