A dog biscuit is a hard biscuit-based dietary supplement for dogs or other canines, similar to human snack food.
Dog biscuits tend to be hard and dry. Dog biscuits may be sold in a flat bone-shape. Some manufacturers claim the dry and hard biscuit texture helps clean the dog's teeth, promoting oral health.
"Dog's bread", made from bran, has been mentioned since at least Roman times. It was already criticized (as in later centuries) as particularly bad bread; Juvenal refers to dog's bread as "filth" - "And bit into the filth of a dog's bread" Et farris sordes mordere Canini.
In Spain, "pan de perro" is mentioned as early as 1623 in a play by Lope de Vega. It is used here in the sense of giving someone blows; to "give dog's bread" to someone could mean anything from mistreating them to killing them. The latter meaning refers to a special bread (also called zarazas) made with ground glass, poison and needles and intended to kill dogs.
The bread meant as food for dogs was also called parruna and was made from bran. This was very likely what was referred to in associating the bread with (non-fatal) mistreatment.
In France, Charles Estienne wrote in 1598: "Take no notice of bran bread,... it is better to leave it for the hunting, or shepherd, or watch dogs." By the nineteenth century, "pain de chien" had become a way of referring to very bad bread: "It is awful, general, they give us dog's bread!"
The English dog biscuit appears to be a nineteenth-century innovation: "With this may be joined farinaceous and vegetable articles — oat-meal, fine-pollard, dog-biscuit, potatoes, carrots, parsnips" (1827); "being in the neighbourhood of Maidenhead, I inspected Mr. Smith's dog-biscuit manufactory, and was surprised to find he has been for a long period manufacturing the enormous quantity of five tons a-week !" (1828)
In the south of England it is much the fashion to give sporting-dogs a food called dog-biscuit instead of barley-meal, and the consequences resulting from this simple aliment are most gratifying. Barley-meal, indeed, is an unnatural food, unless it be varied with bones, for a dog delights to gnaw, and thus to exercise those potent teeth with which nature has furnished him ; his stomach, too, is. designed to digest the hard and tough integument of animal substance; hence, barleymeal, as a principal portion of his subsistence, is by no means to he desired. In small private families it is not always possible to ohtain a sufficiency of meat and bones for the sustenance of a dog, and recourse is too frequently had to a coarse and filthy aliment, which is highly objectionable, especially if the creature be debarred from taking daily exercise, fettered by a chain, and restricted, by situation, from obtaining access to grass ; and no one who has not watched the habits of our faithful allies (as we have done), can be aware of the absolute necessity which exists for his obtaining a constant supply of it. If no other good effect resulted from it than the sleekness of his coat and clearness of his skin, these benefits ought to he procured for him; but when his health and comfort are to be also ensured, who, that has a grain of benevolence in his disposition, would hesitate to perform so simple and gratifying an act of duty?