from
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]