Green’s Dictionary of Slang

1. (US) a sexually aggressive person, usus. male.

1979		‘Violenze Mostruose’ in Storie Blu [trans.] 56: Typical. That old horndog puts his pecker above what’s best for the company.
1985		‘Jennifer Blowdryer’ Modern English 18: horndog (n) Is some creep who is always trying to hunch in on other people’s territory [...] They try to horn in all the time: .
1986		Chapman NDAS 216/2: horndog n armed forces a dedicated fornicator; sexual athlete.
1996		K. Lette Mad Cows 32: I’m denied fresh fruit, late nights, good coffee [...] and hot jungle sex with the horndog Himbo of my choice.
2004		C. Hiaasen Skinny Dip 73: I wouldn’t trust the guy alone with my bowling ball, that’s what a horndog he is.
2008		G. Pelecanos Way Home (2009) 49: A horn dog who tended to turn any conversation toward pussy or his dick.
2013		E. Kurz ‘Pegleg’ in ThugLit Aug. [ebook] [T]he horndog making time with the hooker even them truckers next door wouldn't prod with a stob.
2018		NYRB 18 Jan. 🌐 Playing the horndog with one of her own priestesses was inexcusable.
2021		J. Ellroy Widespread Panic 175: [W]icked wives and horndog hubbies, hot to trot.

2. a sexually frustrated person.

1999

J. Ellroy ‘My Life as a Creep’ in Destination: Morgue! (2004) 127: [In a pornographic bookshop] You paid to browse [...] Horndogs browsed all night.