Green’s Dictionary of Slang

(UK juv.) a school ‘game’ whereby the waistband of the victim’s underwear is grasped and pulled up hard, thus causing them pain.

2001		OnLine Dict. of Playground Sl. 🌐 grundy v. For one or more assailants to seize waistband of selected victim’s underpants and pull up same (sharply) to cause maximum pain and distress.
2006		D. Mitchell Black Swan Green 273: A grundy’s where a bunch of kids yank you up, hard, by your underpants.