The Collaborative International Dictionary of English v.0.48:

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.] (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]