Green’s Dictionary of Slang

portion n.

an act of sexual intercourse.

[UK]Wycherley Love in a Wood (1967) III i: Sir, ’tis a charitable thing to give a young Maid a Portion.
[UK]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’.