Ni idea.

Because it is an elliptical expression: Spanish often leaves out words that are understood from context.

Ni idea is basically a shortened version of No tengo ni idea (I have no idea). In conversation, dropping the rest is very natural.

For example:

• ¿Sabes dónde está Marta?
• Ni idea.

That sounds completely normal in Spain.

On its own, ni usually means nor or not even.

In this expression, it is part of the fixed phrase ni idea, which strengthens the negative idea. You can think of the full version:

• No tengo ni idea.

Here, ni adds emphasis, almost like not the slightest idea.

So in Ni idea, the negative force is understood even though no tengo has been omitted.

Yes, it is informal to neutral conversational Spanish. It is very common in everyday speech in Spain.

If you want something more neutral or polite, you could say:

• No lo sé. = I don’t know.
• No tengo ni idea. = still common, but a bit fuller and often stronger
• No sabría decirte. = more polite / softer, literally I wouldn’t know how to tell you

So Ni idea is fine with friends, classmates, coworkers in casual situations, and in most everyday conversations.

Usually, yes.

• No lo sé = I don’t know
• Ni idea = No idea / I have absolutely no idea

So Ni idea often sounds a bit more emphatic. It suggests total lack of knowledge, not just simple uncertainty.

Normally, no. *No idea is not the natural Spanish expression.

The usual options are:

• Ni idea
• No tengo ni idea

English uses No idea naturally, but Spanish prefers Ni idea in this shortened form.

Yes. That is one of its most common uses.

For example:

• ¿Quién ha llamado?

• Ni idea.

• ¿Dónde aparqué el coche?
• Ni idea.

It works very well as a short reply when the meaning is obvious from the context.

Yes.

You can expand it when you want to say what you know nothing about:

• Ni idea de eso. = No idea about that.
• Ni idea de quién es. = No idea who he/she is.
• Ni idea de dónde está. = No idea where it is/he/she is.

And in the full version:

• No tengo ni idea de eso.
• No tengo ni idea de qué pasó.

So de is very common after this expression.

Because ni idea is a fixed idiomatic expression.

In the full sentence, Spanish says:

• No tengo ni idea.

Not:

• *No tengo una idea for this meaning

The phrase does not work like a literal countable noun here. It functions as an established expression meaning not the slightest idea.

Not rude by itself, but it can sound blunt depending on your tone.

With a friendly tone, it is perfectly normal:

• Ni idea, la verdad.
• Ni idea, lo siento.

If you want to sound softer, especially in Spain, you can add something like:

• Ni idea, la verdad.
• No lo sé, lo siento.
• Pues, ni idea.

So the phrase itself is fine; the tone and context matter.

A simple approximation is:

• ni ee-DE-a

More closely, in standard Spain pronunciation:

• ni i-DE-a

A few points:

• ni sounds like nee
• idea has three syllables: i-de-a
• the d in idea is usually soft between vowels in Spanish

So the whole phrase is roughly:

• nee ee-DE-ah

No. It is widely understood and commonly used across the Spanish-speaking world.

That said, it is also very normal in Spain, so if you are learning Spanish from Spain, you can use it confidently.

The meaning is basically the same, but the style is different:

• Ni idea = shorter, quicker, more conversational
• No tengo ni idea = full sentence, a bit more explicit

In many situations, they are interchangeable:

• ¿Dónde está Ana?

• Ni idea.

• ¿Dónde está Ana?
• No tengo ni idea.

The second one can sound slightly stronger or more complete, but both are very common.