Foot-the-ball
Also known as "Poore Boy's Funne", this is quite an important part of Unseen Academicals.
However, the thing about football - the important thing about football - is that it is not just about football. The noble game is administered and its rules determined by the Ankh-Morpork Football Association, a fairly recently established governing body.
The Sport also appears in Jingo and Reaper Man
Annotation
Bears much more relation to Medieval football than Association football.