Trousers or jeans usually worn by middle aged women or sad freaks that come up above the belly button creating a large lump of fat that spreads from stomach to minge emphasising their 'minge gut'(see definition) or creating the appearance of one. Watch 80's music videos or browse the aisles of Marks and Spencers for examples.

the whale tail is the shape formed when a g-string rides up high over a womans trousers