Rare term for strict secrecy or confidentiality, often in formal or bureaucratic contexts. Modern Japanese more commonly uses 秘密 (ひみつ) or 機密 (きみつ).
The document was treated as confidential and was not disclosed to the public.
Compound of 禁 (prohibition) and 秘 (secret). The exact historical derivation is uncertain; it is a rare Sino-Japanese term.