A sauce for meat or fish. The liquor from oysters boiled with flavourings which might include soy, ketchup or horseradish. Butter and flour to thicken, the oyster meat added back to serve (Mollard 1802, Glasse 1747, Eaton 1822, etc)

Original Receipt in 'The Art of Cookery, Made Plain and Easy' by Hannah Glasse, 1747 (Glasse 1747);

To make oyster-sauce.
TAKE half a pint of large oysters, liquor and all; put them into a sauce-pan, with two or three blades of mace, and twelve whole pepper-corns; let them simmer over a low fire, till the oysters are fine and plump, then carefully with a fork take out the oysters from the liquor and spice, and let the liquor boil five or six minutes; then strain the liquor, wash out the sauce-pan clean, and put the oysters and liquor into the sauce-pan again, with half a pint of gravy, and half a pound of butter just rolled in a little flour. You may put in two spoonfuls of white wine, keep it stirring till the sauce boils, and all the butter is melted.