Is the area under the chin and above the neck considered 'awrah that must be covered in prayer and outside of it, and is not covering it outside of prayer, while knowing its ruling, considered a major sin?

The chin is where the two jawbones meet, and it is part of the face, differing in the obligation to cover it outside of prayer. However, it is not required to cover it during prayer. As for what is beneath the chin, it is not part of the face, and it must be covered both inside and outside of prayer. What appears from it due to the necessity of movement or adjusting something is excused. A woman must strive to submit to the commands of the Sharia (definition) and be wary of Satan beautifying sins for her.