Is it ensure, insure, or assure?
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."
The word has in mod.Eng. a much stronger sense than it had at an earlier period; it has now an implication of insulting contempt which does not in the same degree belong to any of its synonyms, or to the derivative foolish. [OED]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).
There is no foole to the olde foole [Heywood, 1546]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.
mid-14c., "to be foolish, act the fool," from fool (n.). The meaning "to make a fool of" is recorded from 1590s. Also as a verb 16c.-17c. was foolify. Related: Fooled; fooling. Fool around is 1875 in the sense of "pass time idly," 1970s in sense of "have sexual adventures."
"foolish, silly," considered modern U.S. colloquial, but it is attested from early 13c., from fool (n.).
An adept or enthusiast in what is indicated: Lindy was a flying foolRelated Terms
[1920s+; perhaps because the person is devoted to the extent of foolishness]