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.