"trifling, of no value; invalid, futile," c. 1600, from Latin nugatorius "worthless, trifling, futile," from nugator "jester, trifler, braggart," from nugatus, past participle of nugari "to trifle, jest, play the fool," from nugæ "jokes, jests, trifles," a word of unknown origin.
1540s, "type of pantomime dance;" 1580s, "professional comic fool;" 1590s in the general sense "a clown, a joker;" from French bouffon (16c.), from Italian buffone "jester," from buffa "joke, jest, pleasantry," from buffare "to puff out the cheeks," a comic gesture, of echoic origin. Also see -oon.
Ancient Greek words for "clown" were sklêro-paiktês, from paizein "to play (like a child);" or deikeliktas. Greek also had geloiastes "a jester, buffoon" (from gelao "to laugh, be merry"); there was a khleuastes "jester," but it had more of a sense of "scoffer, mocker," from khleuazo "treat with insolence." Other classical words used for theatrical clowns were related to "rustic," "peasant" (compare Latin fossor "clown," literally "laborer, digger," related to fossil).
Coulrophobia looks suspiciously like the sort of thing idle pseudo-intellectuals invent on the internet and which every smarty-pants takes up thereafter; perhaps it is a mangling of Modern Greek klooun "clown," which is the English word borrowed into Greek.