How to Say It's a Secret in Japanese: Himitsu, Naisho and Koko Dake

Four ways to keep it quiet

These four lines cover nearly every everyday secret. Naisho da yo marks something as secret in a friendly way. Himitsu is the general word. Koko dake no hanashi opens a confidence. Dare ni mo iwanaide asks the listener to keep it.

Naisho is often said with a finger to the lips and a smile. Shī, the hushing sound, works in Japanese too, and children love naisho-banashi, whispered secrets.

Himitsu or naisho?

Both mean secret, but they live in different places. Himitsu is a noun with a wide range: a company secret, a secret recipe, the secret of success. Naisho is casual and personal; it is about not telling particular people. Mama ni wa naisho, don't tell Mum, is pure naisho. As a verb phrase, naisho ni suru means to keep something quiet.

Himitsu also forms many compounds: himitsu kichi, a secret den that children build, and himitsu heiki, a secret weapon, used jokingly for a star player or a favourite ingredient.

Opening a confidence and asking for silence

Koko dake no hanashi da kedo, this is just between us, but, is the classic opener for a bit of gossip or news. Jitsu wa, actually, often follows. To ask for silence, dare ni mo iwanaide ne is friendly, zettai, absolutely, makes it firmer, and honnin, the person themselves, lets you name who must not hear.

When someone tells you a secret, reassure them: wakatta, iwanai yo, got it, I won't tell. Then keep your word; trust built this way matters in any friendship.

Playful dodges: sore wa himitsu desu

When someone asks something you would rather not answer, such as your age, the price of a gift or who you are dating, sore wa himitsu desu with a smile is a charming way out. It is not rude; it signals a game, and most people laugh and move on. Naisho desu works the same way and sounds even lighter.

Age is a classic topic for this dodge, because people in Japan often ask it early to work out how to speak to each other. If you do not mind sharing, a number is fine; if you do, sore wa himitsu desu with a laugh keeps things friendly.

Kuchi ga katai: people who keep secrets

Japanese describes discretion with the mouth. Kuchi ga katai, a hard mouth, is someone who keeps secrets, and it is a real compliment. Kuchi ga karui, a light mouth, is someone who blabs. At work, confidential matters are naimitsu (内密) or, for internal documents, shagaihi (社外秘), not for outside the company; these are formal words you will see more than say.

Keeping a confidence at work is a big part of trust, and kuchi ga katai hito is real praise from a manager.

A spoken exchange: planning a surprise

Maria's birthday is next week, and you are planning a surprise party. You tell a mutual friend after class and ask them to keep it quiet.

Honnin, the person themselves, is a handy word whenever you talk about someone who is not present: honnin ni kiite, ask them directly.

Surprises, slips and spoilers

Sapuraizu, surprise, is a popular word for birthday plans, and naisho is its natural partner. Bareru, to be found out, is the casual verb for a secret leaking: barechatta!, it's out!, is what you say when the surprise is ruined, and barenai yō ni, so that nobody finds out, is the planning phrase.

Spoilers have their own word: netabare, from neta, material, and bare, being found out. Netabare shinaide, no spoilers, is useful with friends who are ahead of you in a drama. And when a surprise works, the guest of honour usually cries zenzen kizukanakatta!, I had no idea!

Common mistakes

• Using naisho in formal or business writing. Use himitsu, or naimitsu for confidential matters.
• Saying himitsu desu flatly to a serious question at work. The playful dodge only works in light conversation.
• Forgetting ni in naisho ni suru and dare ni mo iwanaide.
• Saying dare mo iwanaide. You need ni: dare ni mo, to anyone.

A good rule of thumb: naisho for friends and family, himitsu when in doubt, and naimitsu only in formal work settings.