mastiff (n.) Look up mastiff at Dictionary.com
large, powerful breed of dog, early 14c., from Old French mastin "great cur, mastiff" (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 or watchdog. Form in English perhaps influenced by Old French mestif "mongrel."