Green’s Dictionary of Slang

a man who is suffering from a venereal disease.

1823

‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 78: Fire-plug; ordinary people would imagine this to be the F.P. stuck up against many houses, to tell how many feet distant water-plugs may be found, in case of fire; but, by the double, means the otherwise affected young fellows who may have laid out their money badly in the flesh-market.