Cornish (adj.)

county in the far southwest of England, from Old English Cornwalas (891) "inhabitants of Cornwall," literally "the Corn Welsh," from the original Celtic tribal name *Cornowii (Latinized as Cornovii), literally "peninsula people, the people of the horn," from Celtic kernou "horn," hence "headland," from PIE *ker- (1) "horn; head, uppermost part of the body" (see horn (n.)), in reference to the long "horn" of land on which they live. To this the Anglo-Saxons added the plural of Old English walh "stranger, foreigner," especially if Celtic (see Welsh). The Romans knew it as Cornubia; hence poetic Cornubian.

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.