In medieval times, icing—a sprinkling of sugar—was put on top of savoury as well as sweet foods: fish pies, for instance. But the iced cakes we are familiar with today started to emerge in recognizable form in the seventeenth century; in those days, once the sugar had been applied (either directly, or to a layer of beaten egg white), the cake was returned to the oven for a while for the icing to harden. That was still the case in the eighteenth century, when the term icing is first actually recorded, in Elizabeth Raffald's Experienced English Housekeeper (1769): ‘Tarts that are iced require a slow oven, or the icing will be brown.’ And a hundred years later, Mrs Beeton was describing very much the same method. In this century, however, the tendency has been to go for a softer icing, which requires no cooking at all.
The term icing has also in the past been applied to marzipan, as used for topping cakes: Mrs Beeton gives a recipe for this ‘almond icing’.
Of roughly equal antiquity with the term icing is frosting, which is the preferred word in American English. The term icing sugar is first recorded in 1889; American English also uses confectioners' sugar.
The metaphorical use of icing on the cake for ‘desirable extras’ first crops up in print in a 1969 issue of the Listener, but is probably earlier.
Subjects: Cookery, Food, and Drink.