n.; pl. Mastiffs [ Mastives is irregular and unusual. ] [ Prob. fr. Prov. E. masty, adj., large, n., a great dog, prob. fr. mast fruit, and hence, lit., fattened with mast. There is perh. confusion with OF. mestif mongrel; cf. also F. mâtin mastiff, OF. mastin. ] (Zool.) A breed of large dogs noted for strength and courage. There are various strains, differing in form and color, and characteristic of different countries. [ 1913 Webster ]

Mastiff bat (Zool.) , any bat of the genus Molossus; so called because the face somewhat resembles that of a mastiff. [ 1913 Webster ]