1560s, "rustic;" 1580s, "boorish, ungainly, awkward," from clown (n.) + -ish. Meaning "pertaining to or characteristic of a (stage) clown" is perhaps from c. 1600. Related: Clownishly; clownishness.
Entries linking to clownish
clown n.
1560s, clowne, also cloyne, "man of rustic or coarse manners, boor, peasant," a word of obscure origin; the original form and pronunciation are uncertain. Perhaps it is from Scandinavian dialect (compare Icelandic klunni "clumsy, boorish fellow;" Swedish kluns "a hard knob; a clumsy fellow," Danish klunt "log, block"), or from Low German (compare North Frisian klönne "clumsy person," Dutch kloen). OED describes it as "a word meaning originally 'clod, clot, lump', which like those words themselves ..., has been applied in various langs. to a clumsy boor, a lout."
The theory that it is from Latin colonus "colonist, farmer" is less likely, but awareness of the Latin word might have influenced the sense development in English.
Meaning "professional fool, professional or habitual jester" is c. 1600. "The pantomime clown represents a blend of the Shakes[pearean] rustic with one of the stock types of the It[alian] comedy" [Weekley]. Meaning "contemptible person" is from 1920s. Fem. form clowness attested from 1801.
-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.