Although the prawn cocktail gained popularity in the 70s and 80s in the UK, it is thought to have actually originated in the 19th century in California. It is said that a local miner was bored with plain oysters and he started ordering them with ketchup and horseradish, which became and oyster cocktail and then other seafood cocktails started popping up on menus all over California. The name either comes from the fact that it's usually served in a cocktail glass, or it's a cocktail mix of ingredients.