Green’s Dictionary of Slang

undercrackers n.

male underwear.

1997		Roger’s Profanisaurus in Viz 87 Dec. n.p.: undercrackers n. Kecks; shreddies; Y fronts.
2010		Jules Goldsmith Twitter 28 Dec. 🌐 My husband is refusing to change his new grey undercracks. He thinks they are now his lucky cricket pants.
2022		Twitter 15 Aug. 🌐 Went to work today with my undercrackers on back to front.