noun
mentally deranged person; lunatic
A somewhat dated or literary term for someone who has lost their sanity. In modern everyday speech, 狂人 or 精神異常者 are more common.
彼は乱心者のように振る舞った。
He behaved like a lunatic.
Compound of 乱心 (ranshin, 'deranged mind') + 者 (mono, 'person'). The exact historical derivation is uncertain, but the kanji spelling is conventionally associated with the meaning.