Over the nearly 150 years since Angus Buchanan helped win the first international rugby match, discussing Scottish rugby has become one of several covert identifiers for wizards meeting in front of Muggles and seeking to establish each other’s credentials.
-- Essays by Rowling
Thanks to Buchanan, wizards around the world support Scotland for the sport of rugby, whereas they support their own countries for other sports. The wizard fan club is called the Wizarding Supporters of Scottish Rugby Union (W.S.S.R.U.).
The Muggle sport of rugby:
From the Web
Writing by J.K. Rowling on WizardingWorld (Pottermore): Scottish Rugby
Harry Potter Wiki: Scottish rugby team
WizardingWorld (Pottermore) features: Everything you need to know about Squibs