A house- dog of great strength and courage.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
A breed of large, powerful dogs, valued chiefly as watchdogs.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
A large and strong variety of dog.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
A large and strong variety of dog much used as a watchdog.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
A large fierce dog.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)

Usage examples:

Especially noteworthy is his representation here with his favorite mastiff Leo," his inseparable companion.
- The University of Michigan by Wilfred Shaw
His hair was bristling, no longer with fear, but with sheer brute fury; his lips were drawn back from the clenched teeth; his nostrils distended and quivering; his forehead wrinkled like that of an angry mastiff
- Into the Primitive by Robert Ames Bennet
I was immediately admitted into the premises, and even the large mastiff seemed to know that I was not an unexpected visitor- for he neither growled, nor betrayed any symptoms of uneasiness.
- A Bibliographical, Antiquarian and Picturesque Tour in France and Germany, Volume One by Thomas Frognall Dibdin