early 15c., "of or belonging to animals, non-human," from Old French brut "coarse, brutal, raw, crude," from Latin brutus "heavy, dull, stupid, insensible, unreasonable" (source also of Spanish and Italian bruto), said to be an Oscan word, from PIE *gwruto-, suffixed form of root *gwere- (1) "heavy." Before reaching English the meaning expanded to "of the lower animals." Used in English of human beings from 1530s, "wanting in reason, blunt or dull of sentiment, unintelligent." The sense in brute force (1736) is "irrational, purely material."
Brute ... remains nearest to the distinguishing difference between man and beast, irrationality .... Brutish is especially uncultured, stupid, groveling .... Brutal implies cruelty or lack of feeling: as brutal language or conduct. [Century Dictionary]