Quick answer
ni idea
Natural: no idea / no clue
Literal: “not even an idea”
How it is used
—¿Dónde está Luis? —Ni idea.“Where is Luis?” “No idea.”
Ni idea de cocina.I know nothing about cooking.
No tengo ni idea.I have no idea at all.
Ni idea, lo siento.No clue, sorry.
Where it is said
It is used across the Spanish-speaking world; in Mexico <b>quién sabe</b> or the clipped <b>sepa</b> cover the same ground.
A mini dialogue
— ¿A qué hora abre el museo?— What time does the museum open?
— Ni idea, mira la web.— No idea, check the website.
Watch out
ni idea
Keep it for friends and relaxed situations; with a boss or an official the neutral no sé sounds better.
Check yourself
What does “Ni idea” mean?