A personal meeting with the sole intention of having sex. It's Americanism that has made its way over here in part and not widely used especially those of us from a sheltered background. ?

Casual relations.....

In the US its a derogatory sexist term meant to pass judgment and bestow guilt on women who even suggest they just need to hold someone tonight for whatever their personal reason may be without asking for a lifetime commitment from partners as many women are prone to force other women to do. A holdover from religious teaching regarding personal sex lives.

In English or American?
In English it can mean an agreement to meet up, usually regularly. A sort of casual "getting engaged".
In American it tends to mean a more intimate relationship, long or short time, involving "nookie"

having sex.

"hook-up" - generally genitalia meeting genitalia , or some variation thereof