Mastiff \Mas"tiff\, 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. m[^a]tin mastiff, OF. mastin.]

(Zo["o]l.) A breed of large dogs noted for strength and courage. There are various strains, differing in form and color, and characteristic of different countries.

Mastiff bat (Zo["o]l.), any bat of the genus Molossus; so called because the face somewhat resembles that of a mastiff.

Source: Webster's Revised Unabridged Dictionary (1913)