Metiche

The word metiche means 'nosy' but the meaning often goes beyond mere curiosity to actual involvement.

• No seas metiche. Butt out.
• No soy metiche. Me mantengo informado. I'm not nosy. I just like to be well-informed. (telenovela dialogue)
• La metiche de tu hermana. That meddling, busybody sister of yours.

Chismoso

Chismoso, literally, prone to gossiping, is similar to metiche but doesn't imply involvement as strongly. No seas chismoso. Don't be so curious about things that don't concern you. Or: don't go spreading gossip.

¿Por?

When someone asks you an impertinent question, you can often respond with a simple ¿Por?

— ¿Cómo te fue anoche?
— ¿Por? Why do you ask? Don't be nosy.

This strategy also works for when you simply want to better understand the intentions of the questioner in order to know the best way to respond.

— ¿Qué haces? What are you up to?
— Nada. ¿Por? Nothing. Why do you ask?

In these examples, you could instead have responded with the longer ¿Por qué preguntas?