Any material (which could include vinyl) that attempts to cover usually sloppy, roast beef style pussy lips in an attempt to constrict their natural tendency to dangle like the sweating flaps of overexposed and necrotic skin under good ol' granny's arms. Pelican briefs are easily spotted on the beach, where there the tell-tale sign is a wad of festering flesh akin to the underside of a pelican's beak.
"Hey little buddy, look at those pelican briefs..."
"...Oh Skipper, I think I threw up a little..."
"It'll be alright little buddy, let's go home and I'll give you a nice warm sizzle chest you can sport on the boardwalk this evening."
"I love you Skipper....but my stink eye still burns."