Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class IV. Words Relating to the Intellectual Faculties
Division (I) Formation of Ideas
Section V. Results of Reasoning

499. Imbecility. Folly.

   NOUN:IMBECILITY; want of intelligence [See Intelligence. Wisdom], want of intellect [See Intellect]; shallowness, unwisdom, silliness, foolishness &c. adj.; morosis, incapacity, vacancy of mind, poverty of intellect, clouded perception, poor head; apartments -, rooms -, space- to let [all slang]; nobody home [slang]; stupidity, insulsity [rare], stolidity; hebetude, dull understanding, meanest capacity, shortsightedness; incompetence (unskillfulness) [See Unskillfulness].
  BIAS [See Misjudgment]; infatuation (insanity) [See Insanity]; one’s weak side.
  SIMPLICITY, puerility, babyhood; senility, dotage, anility, second childishness, fatuousness, fatuity; idiocy, idiotism, jobbernowlism [colloq., Eng.], driveling, driveling idiocy; senile dementia.
  FOLLY, frivolity, irrationality, trifling, ineptitude, nugacity, futility, inconsistency, lip wisdom, conceit; sophistry [See Intuition. Sophistry]; giddiness (inattention) [See Inattention]; eccentricity [See Insanity]; extravagance (absurdity) [See Absurdity]; rashness [See Rashness].
  act of folly [See Unskillfulness].
   VERB:BE IMBECILE &c. adj.; have no -brains, – sense [See Intelligence. Wisdom]; have a screw loose [colloq.].
  TRIFLE, drivel, radoter [F.], dote; ramble (madness) [See Insanity]; play the -fool, – monkey; take leave of one’s senses; not see an inch beyond one’s nose; stultify oneself [See Unskillfulness]; talk nonsense [See Absurdity].
   ADJECTIVE:[APPLIED TO PERSONS] UNINTELLIGENT, unintellectual, unreasoning; mindless, witless, reasonless, brainless; half-baked [colloq.], having no head [See Intelligence. Wisdom]; not bright [See Intelligence. Wisdom]; inapprehensive, thick [colloq.].
  blockish, unteachable; Bœotian, Bœotic; bovine; ungifted, undiscerning, unenlightened, unwise, unphilosophical; apish, simious, simian.
  weak-, addle-, puzzle-, blunder-, muddle- [colloq.], jolter-, jolt-, chowder-, pig-, beetle-, buffle- [obs.], chuckle-, mutton-, gross- headed; maggot-pated [obs.], beef-headed, beef-witted, fat-headed, fat-witted.
  WEAK-MINDED, feeble-minded; dull-, shallow-, lack- brained; rattle- brained, -headed; sap-head [colloq.], muddy- brained, addle-brained; half-, lean-, short-, shallow-, dull-, blunt- witted; shallow-, clod-, addle- pated; dim-, short- sighted; thick-skulled; thick-headed; weak in the upper story [colloq.], inapprehensible, nutty [slang], batty [slang], balmy in the crumpet [slang], loony or luny [slang].
  SHALLOW, borné [F.], weak, wanting, soft [colloq.], sappy, spoony or spooney [slang]; dull, – as a beetle.
  STUPID, heavy, insulse [rare], obtuse, blunt, stolid, doltish; asinine; inapt [See Unskillfulness]; prosaic [See Dullness]; hebetudinous, hebetate, hebete [rare].
  CHILDISH, childlike; infantine, infantile, babyish, babish; puerile, senile, anile; simple(credulous) [See Credulity]; old-womanish.
  IMBECILE, fatuous, idiotic, driveling; blatant, babbling; vacant; sottish; bewildered [See Uncertainty].
  FOOLISH, silly, senseless, irrational, insensate, nonsensical, inept; maudlin.
  NARROW-MINDED [See Misjudgment]; bigoted (obstinate) [See Obstinacy]; giddy (thoughtless) [See Inattention]; rash [See Rashness]; eccentric (crazed) [See Insanity].
  [APPLIED TO ACTIONS] FOOLISH, unwise, injudicious, improper, unreasonable, without reason, ridiculous, silly, stupid, asinine; ill-imagined, ill-advised, ill-judged, ill-devised; mal entendu [F.]; inconsistent, irrational, unphilosophical; extravagant (nonsensical) [See Absurdity]; sleeveless [obs.], idle; useless [See Inutility]; inexpedient [See Inexpedience]; frivolous (trivial) [See Unimportance].
  1. Davus sum non Ædipus.
  2. A fool’s bolt is soon shot.—Henry V
  3. Clitellæ bovi sunt impositœ.—Cicero
  4. Fools rush in where angels fear to tread.—Pope
  5. Il n’a ni bouche ni éperon.
  6. The bookful blockhead, ignorantly read.—Pope
  7. To varnish nonsense with the charms of sound.—Churchill
  8. And duller should’st thou be than the fat weed That roots itself in ease on Lethe wharf.—Hamlet
  9. Men are so necessarily foolish that not to be a fool is merely a varied freak of folly.—Pascal