Etymology
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folly (n.)
early 13c., "mental weakness; foolish behavior or character; unwise conduct" (in Middle English including wickedness, lewdness, madness), from Old French folie "folly, madness, stupidity" (12c.), from fol (see fool (n.)). From c. 1300 as "an example of foolishness;" sense of "costly structure considered to have shown folly in the builder" is attested from 1650s. But used much earlier, since Middle English, in place names, especially country estates, probably as a form of Old French folie in its meaning "delight." Related: Follies.
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follies (n.)
"glamorous theatrical revue with lots of pretty girls," 1880, from French folies (mid-19c.), from folie (see folly), probably in its sense of "extravagance" (compare extravaganza).
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*bhel- (2)
Proto-Indo-European root meaning "to blow, swell," "with derivatives referring to various round objects and to the notion of tumescent masculinity" [Watkins].

It forms all or part of: bale (n.) "large bundle or package of merchandise prepared for transportation;" baleen; ball (n.1) "round object, compact spherical body;" balloon; ballot; bawd; bold; bole; boll; bollocks; bollix; boulder; boulevard; bowl (n.) "round pot or cup;" bulk; bull (n.1) "bovine male animal;" bullock; bulwark; follicle; folly; fool; foosball; full (v.) "to tread or beat cloth to cleanse or thicken it;" ithyphallic; pall-mall; phallus.

It is the hypothetical source of/evidence for its existence is provided by: Greek phyllon "leaf," phallos "swollen penis;" Latin flos "flower," florere "to blossom, flourish," folium "leaf;" Old Prussian balsinis "cushion;" Old Norse belgr "bag, bellows;" Old English bolla "pot, cup, bowl;" Old Irish bolgaim "I swell," blath "blossom, flower," bolach "pimple," bolg "bag;" Breton bolc'h "flax pod;" Serbian buljiti "to stare, be bug-eyed;" Serbo-Croatian blazina "pillow."

An extended form of the root, *bhelgh- "to swell," forms all or part of: bellows; belly; bilge; billow; bolster; budget; bulge; Excalibur; Firbolgs.

An extended form of the root, *bhleu- "to swell, well up, overflow," forms all or part of: affluent; bloat; confluence; effluent; effluvium; efflux; fluctuate; fluent; fluid; flume; fluor; fluorescence; fluoride; fluoro-; flush (v.1) "spurt, rush out suddenly, flow with force;" fluvial; flux; influence; influenza; influx; mellifluous; phloem; reflux; superfluous.
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niceness (n.)

1520s, "folly, foolish behavior," from nice (q.v.) + -ness. Meaning "exactness" is from 1670s; that of "pleasantness" is from 1809.

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fatuity (n.)

1640s, from French fatuité (14c.), from Latin fatuitatem (nominative fatuitas) "foolishness, folly," from fatuus "foolish, insipid" (see fatuous).

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saeva indignatio 

Latin phrase from Swift's epitaph; "savage indignation;" an intense feeling of contemptuous anger at human folly.

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giddiness (n.)
late 13c., "thoughtless folly, flightiness," from giddy + -ness. Meaning "dizziness, vertigo" is from late 14c.
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dizziness (n.)

Old English dysignesse, "folly;" see dizzy + -ness. From c. 1400 as "giddiness, whirling in the head."

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insipience (n.)
early 15c., "lack of wisdom, foolishness," from Old French insipience (15c.) or directly from Latin insipientia "folly, unwisdom," from insipientem "unwise, foolish" (see insipient).
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madness (n.)

late 14c., "insanity, lunacy, dementia; rash or irrational conduct, headstrong passion, extreme folly," from mad (adj.) + -ness. Sense of "foolishness" is from early 15c.

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