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Definition of lunatic : 1. A person affected by lunacy; an insane person, esp. one who has lucid intervals; a madman; a person of unsound mind.
2. Affected by lunacy; insane; mad.
3. Of or pertaining to, or suitable for, an insane person; evincing lunacy; as, lunatic gibberish; a lunatic asylum.

Synonyms:

mental, weak-minded, harebrained, ruinous, simpleminded, non compos mentis, barmy, sane, loony, fool, cabbage, nutjob, fruity, asinine, swashbuckler, damaging, unearthly, oddball, unwise, nuts, daredevil, bats, absurd, softheaded, dotty, dopey, madcap, tomfool, buggy, loony tunes, crackers, touched, imbecile, dwarf, weirdie, wud, disastrous, harum-scarum, madwoman, wise, eccentric, hazardous, bananas, loco, knowledge, brainsick, cuckoo, perilous, backward, scatty, half-baked, unhinged, imbecilic, haywire, bughouse, risky, weirdo, unsafe, cockeyed, disordered, brainless, daffy, off, silly, nonsensical, cripple, hothead, fire-eater, hunchback, wrong, gaga, foolish, crackbrained, loon, screwball, sappy, dingbat, lunkheaded, wacko, headcase, bubbleheaded, featherheaded, oddity, witless, cranky, dippy, wacky, preposterous, certifiable, crazed, inept, balmy, humpback, screwy, bonkers, moonstruck, harmful, senseless, moronic, loopy, half-witted, zany, meshuga, space cadet, ding-a-ling, unsound, distraught, bedlam, mentally ill, fatuous, jerky, injurious, cracked, dangerous, ability

foolish (part of speech: adjective)

idiotic, daft, stupid

lunatic (part of speech: noun)

fiend, crackpot, maniac, kook, madman, nut, fiendish, crank, psychopath

insane (part of speech: adjective)

demented, maniacal, irrational, feverish, babbling, kooky, hysterical, deranged, crack-brained, brain-sick, psychotic, nutty, mad, unbalanced, berserk, crazy, frenzied, delirious, neurotic, schizoid, psycho, blithering, batty, insane

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