Green’s Dictionary of Slang

bold thing, the n.

(Irish) sexual intercourse.

1986

T. McDonagh My Green Age n.p.: I told her athletes weren’t much good at the bould thing. They used up their juices in other ways [BS].

In phrases

do the bold thing (v.)

(Irish) to have sexual intercourse.

1965		T. Gray Starting from Tomorrow 45: [He] had succeeded in doing the bold thing with the Editor’s aging secretary on a pile of dusty papers just inside the door.
1996		Irish Times 22 Oct. n.p.: A group of pro-chastity campaigners are roving round the country in an effort to dissuade young people [...] from doing the bold thing until marriage [BS].
2000		P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 9: [...] us doing the bould thing out in her gaff in Clonskeagh.