Green’s Dictionary of Slang

1. simulated or mutually unsatisfying intercourse.

1962		(con. 1940s) G. Mandel Wax Boom 115: ‘Tuned in on the radio [...] Axis Sally was talking one of her dry humps at us.’ ‘Shit,’ Gingold said. ‘Exactly what I mean. She wants us to grab our spouts and forget the Krauts.’.
2007		C. Goffard Snitch Jacket 14: A hideous nightly dry-hump, lacking passion or even pleasure.

2. in fig. use, a failure, a disappointment; a pointless exercise.

1992		R. Marcinko Rogue Warrior (1993) 289: We went. But Viques, too, turned out to be yet another dry hump.
2000		J. Ellroy ‘Balls to the Wall’ in Destination: Morgue! (2004) 26: Heist. Dry hump. Fucking. Misappropriation.
2011		E. Beetner ‘Zed’s Dead, Baby’ in Pulp Ink [ebook] The whole thing was a bit of a dry hump, but damn if I didn’t feel charged up as hell.
2023		(con. 1962) J. Ellroy Enchanters 277: It’s nothing but a dry hump until the Creep snuffs Marilyn.