Origin and history of oafish

1620s, auf, oph (modern form from 1630s; oafish is from 1610s), "a changeling; a foolish or otherwise defective child left by the fairies in place of another carried off," from a Scandinavian source such as Norwegian alfr "silly person," in Old Norse "elf" (see elf). Hence, "a misbegotten, deformed idiot, a simpleton" (17c.). Until recently, some dictionaries still gave the plural as oaves.

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.