"uncouth, uncultured, rustic, so low-bred in habits as to be offensive," 1560s, from boor (n.) + -ish. Related: Boorishly; boorishness.
Entries linking to boorish
boor n.
early 14c., "country-man, peasant farmer, rustic," from Old French bovier "herdsman," from Latin bovis, genitive of bos "cow, ox." This was reinforced by or merged with native Old English gebur "dweller, farmer, peasant" (unrelated but similar in sound and sense), and 16c. by its Dutch cognate boer, from Middle Dutch gheboer "fellow dweller," from Proto-Germanic *buram "dweller," especially "farmer" (compare German Bauer), from PIE root *bheue- "to be, exist, grow."
"A word of involved history in and out of English, though the ultimate etymology is clear enough" [OED]. In English it often was applied to agricultural laborers in or from other lands, as opposed to the native yeoman; the negative transferred sense of "one who is rude in manners," attested by 1560s (in boorish), is from the city-dweller's notion of clownish rustics. Related: Boorishness.
-ish
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.