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."
It forms all or part of: amanuensis; command; commando; commend; countermand; demand; Edmund; emancipate; legerdemain; maintain; manacle; manage; manciple; mandamus; mandate; manege; maneuver; manicure; manifest; manipulation; manner; manque; mansuetude; manual; manubrium; manufacture; manumission; manumit; manure; manuscript; mastiff; Maundy Thursday; mortmain; Raymond; recommend; remand; Sigismund.
It is the hypothetical source of/evidence for its existence is provided by: Hittite maniiahh- "to distribute, entrust;" Greek mane "hand," Latin manus "hand, strength, power over; armed force; handwriting," mandare "to order, commit to one's charge," literally "to give into one's hand;" Old Norse mund "hand," Old English mund "hand, protection, guardian," German Vormund "guardian;" Old Irish muin "protection, patronage."
masc. proper name, from German Bernhard, literally "bold as a bear," from Old High German bero "bear" (see bear (n.)) + harti "hard, bold, strong" (from PIE root *kar- "hard"). Saint Bernard (1091-1153) was the famous Cistercian monk; the breed of Alpine mastiff dogs is said to have been so called from early 18c. (in English by 1839), because the monks of the hospice named for him in the pass of St. Bernard (between Italy and Switzerland) sent them to rescue snowbound travelers.