1560s, clowne, also cloyne, "rustic, boor, peasant," origin uncertain. Perhaps from Scandinavian dialect (cf. Icelandic klunni "clumsy, boorish fellow;" Swedish kluns "a hard knob; a clumsy fellow," Danish klunt "log, block"), or akin to North Frisian klönne "clumsy person." Or, less likely, from Latin colonus "colonist, farmer," though awareness of this 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. comedy" [Weekley]. Meaning "contemptible person" is from 1920s. Fem. form clowness attested from 1801.