this (person/thing)

In the example above, the speaker uses "this" instead of "a". There's a small difference between the two. When you say "this ___" it sounds like you're talking about one specific thing. "A ___" is more general. 

You use "this ___" instead of "a" or "the" when you're introducing something specific that you're going to talk about. It's usually used at the beginning of a story or explanation. For example:

Amy and I were sitting there talking and this guy I know walked by.

I've had this song stuck in my head for two days!

Today I was talking with this girl at work who just got back from maternity leave.

In all of these examples, the speaker will continue by telling a story about that topic.

People only use "this ___" in this way in casual spoken English.

(someone) is nosy

A "nosy" person is someone who wants to know private information about other people. It has a negative association, because "nosy" people are annoying.

People that are often "nosy" include neighbors and family members.

(someone) next door

A person who's "next door" lives in the house or apartment right next to yours.

Have you met the guy next door?

(someone) is in (someone's) business

Being "in someone's business" means getting involved in their personal lives.

For example, if you've tried to learn or give advice about a person's:

• finances
• love life
• health issues

...then you're "in their business".

This is why, when someone is getting too personal, you say:

Get out of my business!