Origin and history of brutish

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"]

adjectival word-forming element, Old English -isc "of the nativity or country of," in later use "of the nature or character of," from Proto-Germanic suffix *-iska- (cognates: Old Saxon -isk, Old Frisian -sk, Old Norse -iskr, Swedish and Danish -sk, Dutch -sch, Old High German -isc, German -isch, Gothic -isks), cognate with Greek diminutive suffix -iskos. In its oldest forms with altered stem vowel (French, Welsh). The Germanic suffix was borrowed into Italian and Spanish (-esco) and French (-esque). Colloquially attached to hours to denote approximation, 1916.

The -ish in verbs (abolish, establish, finish, punish, etc.) is a mere terminal relic from the Old French present participle.