Green’s Dictionary of Slang

a venereal discharge.

1655		Mercurius Fumigosus 50 9–16 May 396: [They will supply] Oysters ... to strengthen the backs of the Horse-coursers, that are Ranke Riders, In requitall whereof, they have promised to finde them a Teame of Running Naggs, to help them home with their Ware, which Running Naggs shall be so fleet, that they shall run faster then a Winchester Goose can flie.
1668		Head Eng. Rogue Pt 2 340: Keeping an open stable for all Horses, gave entertainment to a running Nag, which so paid her, that she was soundly pepper’d.
1674		T. Duffet Epilogue Spoken by Heccate and Three Witches 32: A running Nag I’le thee lend.
1679–80		‘Jennies Answer’ in Chappell Roxburghe Ballads (1874) II 154: Sawny rid home on a Running Nagg, and ... never was Scot in such a case with riding upon a London Jade. But now he repents o’th’ Painted-face, and bans the lewd Queans of London fine He fain wou’d have let his Nag run a race, But Sawny shall never more be mine.
1683		Whores Rhetorick 178: If you would desire to maintain your Body in good order, you must be nicely cautious not to allow any commerce with Men noted to fly at all Games, and famous for never wanting a Neapolitan running-Nag.
1696		Motteux (trans.) Pantagruelian Prognostications (1927) II 694: Let them beware of the crinkams [...] buboes, or running-nags.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Running horse, or nag, a clap, or gleet.
1811		Lex. Balatronicum [as cit. 1785].
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.