After a prolonged bout of anal sex, the giving party turns the recieving party around and proceeds to thwack them on the forehead with their penis/dildo leaving a brown mushroom in its stead. Also known as the brown mushroom to go, because one does not want to stick around after giving a brown mushroom.
notes: brown mushroom is only theoretical, poo does not actually stick to the penis in quantities that would make this act possible.
