Green’s Dictionary of Slang

1. underpants.

1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: shreddies n. Undercrackers; kecks; underpants; trolleys.
2000		N. Griffiths Grits 419: Oof! Christ thill be some skiddies in yer shreddies after tha one bigad. Christ that’s fuckin disgustin.

2. (UK juv.) a ‘game’ whereby the victim’s underpants are tugged upwards so hard that they tear.

2001

OnLine Dict. of Playground Sl. 🌐 shreddies n. extreme wedgie: the act of several boys gathering round a chosen victim, reaching into said victims waistband and all hauling up victim underpants until they ripped.