Green’s Dictionary of Slang

an adulterer; thus (W.I.) horner-man.

1533		J. Heywood Play of Weather in Farmer (1905) 101: A Gentleman, in yonder corner, / And, as I think, his name is Master Horner / A hunter he is, and cometh to make you sport.
1637		Fletcher Elder Brother IV iv: I shall have some musicke yet At my making free oth’ Company of Horners.
1690		D’Urfey Collin’s Walk canto 3 98: The jury weighing the disgraces [...] gave with Sence adorn’d, Not to the Horner, but the horn’d; Attoning by a swinging Sum The unpardon’d sin of Cuckoldom.
1996		Allsopp Dict. Carib. Eng. Usage 297/1: horner-man [...] A man with a reputation for committing adultery.