WordReference Random House Learner's Dictionary of American English © 2020
fool•ish /ˈfulɪʃ/USA pronunciation   adj. 
  1. resulting from or showing a lack of good sense:a foolish prank.
  2. lacking forethought or caution;
    appearing like a fool:felt foolish asking such a big favor.
fool•ish•ly, adv. 
fool•ish•ness, n. [uncountable]

WordReference Random House Unabridged Dictionary of American English © 2020
fool•ish  (fo̅o̅lish),USA pronunciation adj. 
  1. resulting from or showing a lack of sense;
    unwise:a foolish action, a foolish speech.
  2. lacking forethought or caution.
  3. trifling, insignificant, or paltry.
foolish•ly, adv. 
foolish•ness, n. 
  • Middle English folish, foolish. See fool1, -ish1 1250–1300
    • 1, 2.See corresponding entry in Unabridged senseless, vacant, vapid, simple, witless.
      Foolish, fatuous, silly, inane, stupid, asinine imply weakness of intellect and lack of judgment.
      Foolish implies lack of common sense or good judgment or, sometimes, weakness of mind:a foolish decision; The child seems foolish.Fatuous implies being not only foolish, dull, and vacant in mind, but complacent and highly self-satisfied as well:fatuous and self-important; fatuous answers.Silly denotes extreme and conspicuous foolishness;
      it may also refer to pointlessness of jokes, remarks, etc.:silly and senseless behavior; a perfectly silly statement.Inane applies to silliness that is notably lacking in content, sense, or point:inane questions that leave one no reply.Stupid implies natural slowness or dullness of intellect, or, sometimes, a benumbed or dazed state of mind;
      it is also used to mean foolish or silly:well-meaning but stupid;
      rendered stupid by a blow;
      It is stupid to do such a thing.
      Asinine originally meant like an ass;
      it applies to witlessly stupid conversation or conduct and suggests a lack of social grace or perception:He failed to notice the reaction to his asinine remarks.
    • 1.See corresponding entry in Unabridged imprudent, unreasonable, foolhardy, irrational;
      thoughtless, nonsensical, ridiculous, absurd, pointless, preposterous.

'foolishness' also found in these entries (note: many are not synonyms or translations):
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