A person’s bottom.
More example sentences
- Two goals in three minutes, gave the game the firework up the jacksy it required, while also killing it off as a realistic contest.
- Unless they elect the wrong kind of government, in which case they will find American fingers crawling so far up their jacksies that a distinct lump will appear in their throats.
- You know, I really, really hate getting shot, and had I been in a position to do so (and not writhing on the floor bleeding slightly) I would have told Matty so, before inserting his pistol up his jacksie, sideways.
late 19th century: diminutive of jack1.