apish adj.

Old English apa (fem. ape) "an ape, a monkey," from Proto-Germanic *apan (source also of Old Saxon apo, Old Norse api, Dutch aap, German affe), probably a borrowed word, perhaps from Celtic (compare Old Irish apa, Welsh epa) or Slavic (compare Old Bohemian op, Slovak opitza), and the whole group probably is ultimately from an Eastern or non-Indo-European language.

The common word in English until the emergence of monkey in 16c. More technically, in zoology, "a simian; a tail-less, man-like monkey," by 1690s. The only native apes in Europe are the Barbary apes of Gibraltar, intelligent and docile, and these were the showman's apes of the Middle Ages. Apes were noted in medieval times for mimicry of human action, hence, perhaps, the other figurative use of the word, to mean "a fool" (c. 1300).

To go ape "go crazy" is by 1953 (unsanitized or emphatic go apeshit is by 1954), American English; early attestations suggest armed forces slang. To lead apes in hell (1570s) was the fancied fate of one who died an old maid. Middle English plural was occasionally apen. Middle English also had ape-ware "deceptions, tricks."

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.