Green’s Dictionary of Slang

sexual intercourse, often adulterous, enjoyed around lunchtime.

1964		H.G. Brown Sex & Office 122: This institution of the two-hour ‘little affair’ [...] is as old as anyone can remember [...] In the South it’s called a ‘nooner’.
1978		A. Maupin Tales of the City (1984) 234: Why don’t we rent a room at the Mark Twain and have a nooner?
1982		G.V. Higgins Patriot Game (1985) 37: You got some whore lined up for nooners I assume, and then after the bump it’s back to the grind?
1999		Roger’s Profanisaurus 3 in Viz 98 Oct. 21: nooner n. See funch.
2004		J. Ellroy ‘Hollywood Fuck Pad’ in Destination: Morgue! (2004) 207: We need a fuck pad. We’ve got 14 married cops who need a crib for parties and nooners.
2016		‘Chelsea G. Summers’ in Hazlitt.net 8 Jan. 🌐 You tell her you’re turned on and want a nooner, and she can make an informed decision.