late 13c., "silly or stupid person," from Old French fol "madman, insane person; idiot; rogue; jester," also "blacksmith's bellows," also an adjective meaning "mad, insane" (12c., Modern French fou), from Latin follis "bellows, leather bag" (see follicle); in Vulgar Latin used with a sense of "windbag, empty-headed person." Cf. also Sanskrit vatula- "insane," literally "windy, inflated with wind."
Meaning "jester, court clown" first attested late 14c., though it is not always possible to tell whether the reference is to a professional entertainer or an amusing lunatic on the payroll. As the name of a kind of custard dish, it is attested from 1590s (the food also was called trifle, which may be the source of the name).
Feast of Fools (early 14c.), from Medieval Latin festum stultorum) refers to the burlesque festival celebrated in some churches on New Year's Day in medieval times. Fool's gold "iron pyrite" is from 1829. Fool's paradise "state of illusory happiness" is from mid-15c. Foolosopher, a most useful insult, turns up in a 1549 translation of Erasmus. Fool's ballocks is described in OED as "an old name" for the green-winged orchid.