Mastiff definition

An aberrant derivation from Old French mastin (modern mâtin), from Vulgar Latin *mansuetīnus ‘tamed (animal)’, from Latin mansuetus ‘tamed’.

One of several large breeds of dog (such as bulldogs and Saint Bernards), often used as guard dogs

1605: William Shakespeare, King Lear, Act III, Scene VI

Avaunt, you curs! Be thy mouth or black or white, Tooth that poisons if it bite; Mastiff, greyhound, mongrel grim, Hound or spaniel, brach or him.
1922, Virginia Woolf, Jacob's Room, Vintage Classics, paperback edition, page 98

A mastiff lay extended against a battered door.