Green’s Dictionary of Slang

scrump v.2

[? scrumptious adj.]
(US campus)

1. to have sexual intercourse; to neck.

[US]Eble Campus Sl. Mar. 5: scrump – to neck.
[US]Eble Sl. and Sociability 38: Although blends are fashionable in the marketplace, college slang makes little use of this process of word formation, with only about a dozen examples collected since 1972: [...] scrump (screw + bump) ‘have sex’.
OnLine Dict. of Playground Sl. 🌐 scrump v. to have sex, e.g. ‘I’m gonna get my scrump tonight!’.

2. to engage in an adulterous sexual relationship.

implied in scrumper

In derivatives

scrumper (n.)

(US campus) an adulterer.

[UK]Indep. Rev. 20 Jan. 7: Most scrumpers grow out of it in the end, anyway. Much of the fun of scrumping is the forbidden nature of it.