Right in the middle of something; in the midst or height of an event or action—indicates that something is actively happening.
会議は今まっ最中なので、後でかけ直します。
The meeting is in progress right now, so I'll call back later.
試験勉強のまっ最中で、友達からの誘いを断った。
I declined my friend's invitation because I'm in the middle of studying for exams.
工事は真っ最中で、近くの道が通行止めになっている。
Construction is underway, and the nearby road is closed.