be in the middle ofUsed after a noun or activity to explain that someone is currently in that state or engaged in that activity. なんだ adds an explanatory, conversational tone; the subject is often omitted.
currently ...中 indicates that someone is in the middle of an activity or in a particular status, while なんだ adds a casual explanatory tone. The phrase commonly follows nouns such as 仕事中 (at work) or 失業中 (unemployed).