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