A mastiff is a large, powerful, short-haired dog. a large, strong dog, often used to guard houses (mastin, from mansuetus ). 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