Green’s Dictionary of Slang

an act of sexual intercourse.

1672		Wycherley Love in a Wood (1967) III i: Sir, ’tis a charitable thing to give a young Maid a Portion.
1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: portion n. A good seeing to; a shag. As in: ‘I’d give her a portion, and no mistake’.