中なんです

currently ...

→ 中
phrase Rare #40,812
currently ... 中 indicates that something is ongoing or in progress, while なんです adds an explanatory tone. It is often used to explain one's current situation politely.
JA: 今、仕事中なんです。
EN: I'm currently at work.

電話中なんです

I'm on the phone.

減量中なんです

I'm trying to lose weight.

兄は失業中なんです

My older brother is unemployed.

禁煙中なんです

I'm trying to quit smoking.