'Fasik Akbar' refers to major immorality or grave sins in Islam. These are actions that significantly deviate from Islamic principles and can lead to severe consequences. Examples of Fasik Akbar include things like disbelief (kufr), associating partners with Allah (shirk), murder, consuming riba (interest), and intentionally abandoning prayer. Such acts can jeopardize one's faith and spiritual well-being, requiring sincere repentance to seek forgiveness.