mastiff (n.) Look up mastiff at Dictionary.com
large, powerful breed of dog, early 14c., from Old French mastin (Modern French mâtin) or Provençal mastis, both from Vulgar Latin *mansuetinus "domesticated, tame," from Latin mansuetus "tame, gentle" (see mansuetude). Probably originally meaning a dog that stays in the house, thus a guard-dog. Form in English perhaps influenced by Old French mestif "mongrel."