A person who is a bad actor, actor who indicates, over-the-top, could potentially be overacting to make moments larger than they honestly feel about them.
A shitty acting performance, or a schmuck who is unsuccessfully trying to act
Schmuck + acting = schmacting
The actor had no connection to the work, it was obvious he was schmacting
Watching that scene was so painful, the actor clearly had no idea how they truly felt and was indicating an emotion, clearly a very bad case of schmacting
You think that was a performance? Acting, schmacting