Etymology
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mastiff (n.)

large, powerful breed of dog, apparently dating to ancient times, valued as a watch-dog, mid-14c., from Old French mastin "great cur, mastiff" (Modern French mâtin) or Provençal mastis, both of which probably are from Vulgar Latin *mansuetinus "domesticated, tame," from Latin mansuetus "tame, gentle" (see mansuetude). The etymological sense, then, would be a dog that stays in the house, thus a guard-dog or watchdog. The form in English perhaps was influenced by Old French mestif "mongrel."

updated on December 02, 2018

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Definitions of mastiff from WordNet

mastiff (n.)
an old breed of powerful deep-chested smooth-coated dog used chiefly as a watchdog and guard dog;
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.