 It in turn derives from the inverted Y-shape formed by the seams at the front of the underpants.The colloquialism is used even when the fly opening may differ in style, and not actually form the shape of the inverted letter "Y" fly on Coopers/Jockey brand briefs.In Australia, briefs are referred to as "jocks" but should not be confused with jockstraps (more specifically used by athletes) which expose the buttocks.