Lunacy is acting silly or stupid. Your mom might yell, "It's lunacy to go out in the snow wearing those flip flops!"

Some people use the noun lunacy when they talk about actual mental instability, although this is considered old fashioned and outdated. It's much more common to describe the lunacy of someone who does something ridiculous or foolish. While adopting a cat from the animal shelter might be a great idea, adopting all thirty cats from the shelter would be lunacy. Lunacy comes from lunatic, originally "affected with periodic insanity, dependent on phases of the moon."

Definitions of lunacy
  1. noun
    foolish or senseless behavior
    synonyms: craziness, folly, foolery, indulgence, tomfoolery
    see moresee less
    meshugaas, mishegaas, mishegoss
    (Yiddish) craziness; senseless behavior or activity
    buffoonery, clowning, frivolity, harlequinade, japery, prank
    acting like a clown or buffoon
    schtick, schtik, shtick, shtik
    (Yiddish) a prank or piece of clowning
    type of:
    caper, frolic, gambol, play, romp
    gay or light-hearted recreational activity for diversion or amusement
  2. noun
    obsolete terms for legal insanity
    synonyms: insaneness, madness
    see moresee less
    type of:
    relatively permanent disorder of the mind
Word Family

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