How to say "lunatic" in Japanese

• 狂人きょうじんCommon
  no-adjectiveA lunatic or madman: it denotes a person whose mind is deranged. Use it when you simply mean 'a madman' and do not intend to insult.
  その囚人しゅうじん達たちはまるで狂人きょうじんのように互たがいに争あらそった。↗

  The prisoners fought one another like so many mad people.
• キチガイCommon
  nounIt covers madness, insanity and, for a person, 'lunatic' or 'madman', and is normally written in kana. Derogatory and offensive, it amounts to a strong insult when aimed at a person.
  デルバートは気違きちがいだと思おもうよ。↗

  I think that Delbert is crazy.

For a lunatic as a person, 狂人 is the direct equivalent ('madman'); 「キチガイ」 also covers madness or insanity, but applied to a person it is a derogatory insult to be avoided.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 狂人きょうじんlunatic
• きちがいmadness
• ルナティックlunatic
• 乱心者らんしんものmentally deranged person
• メンヘラnutjob