mastiff, breed of very large, powerful working dog developed in England more than 2,000 years ago. It stands from 27 to 33 in. (68.6-83.8 cm) high at the shoulder and weighs from 165 to 185 lb (74.9-83.9 kg). Its coarse, short, close-lying coat may be silver fawn, apricot, or dark fawn brindle in color, with a black muzzle, nose, and ears and black around the eyes. The mastiff was first bred as a fighting dog and guardian. As a fighter it was cited for its physical prowess and courage by Caesar in his account of the Roman invasion of Britain in 55 B.C. Indeed, it was later imported to Rome to fight in the arena. In its native country the mastiff was a popular antagonist in bullbaiting and bearbaiting contests and in organized dogfights until these blood sports were outlawed in 1835. However, throughout the entire history of the breed in England its greatest popularity has derived from its widespread use as a guardian of home and family. This centuries-old association with man is undoubtedly responsible for the mastiff's unexcelled suitability for the role of family companion and its particular devotion to and gentleness with children. The term mastiff is also applied to a general type of giant dog whose origin has been traced to Asia and of which the modern Tibetan mastiff, infrequently seen in the United States, is representative. See dog.

bull mastiff, breed of powerful working dog developed in England in the second half of the 19th cent. It stands from 24 to 27 in. (61-68.6 cm) high at the shoulder and weighs from 100 to 130 lb (45.4-59 kg). Its dense, short coat may be fawn, red, or brindle, with a darker shading on the ears and muzzle. Because of the increasing need to protect game preserves and large estates from poachers, English gamekeepers began to cross existing breeds in an attempt to produce a dog that would possess the required speed, strength, aggressiveness, good night vision, and the capacity to remain silent at the approach of the poacher. It would be the task of the desired dog to knock down the intruder and keep him down until he was captured, rather than simply alarming him into running away. After many breeds were tried, mastiff and bulldog stock were crossed, producing a dog with all the necessary qualities, the bull mastiff. Today it is raised as a guard and show dog and as a pet. See dog.

Breed of powerful but gentle dog of Europe and Asia dating to 3000 BC. Mastiffs fought bears, lions, tigers, bulls, and gladiators in Roman arenas and were used in English bull- and bear-baiting rings. The mastiff stands 28–30 in. (70–75 cm) tall and weighs 165–185 lbs (75–85 kg). It has a broad head, short dark muzzle, and dark drooping ears. Its short coat is apricot, silver fawn, or brindled. The bullmastiff, a bulldog-mastiff crossbreed standing 24–27 in. (61–69 cm) tall and weighing 100–130 lbs (45–59 kg), is used as a police and guard dog.

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