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]
1610s, "a beast" (as distinguished from a man), especially one of the higher quadrupeds, from brute (adj.). From 1660s as "a brutal person, a savage in disposition or manners."
Don't object "Why call him friend, then?" Power is power, my boy, and still
Marks a man,—God's gift magnific, exercised for good or ill.
You've your boot now on my hearth-rug, tread what was a tiger's skin:
Rarely such a royal monster as I lodged the bullet in!
True, he murdered half a village, so his own death came to pass;
Still, for size and beauty, cunning, courage—ah, the brute he was!
[Browning, from "Clive"]