Ears still ringing from his last missionary endeavor?
Takfir is generally translated as "departure from Islamic practices", usually in the sense of ceasing to be a Muslim
Hijira literally means "journey". The Quran uses the word Hijra to mean moving from a bad place or state of affairs to a good one
Since one of the main tactics of the group is to stop performing Islamic practices in order to better blend into non-Muslim societies, and not attract attention, an alternate translation of al-Takfir wa al-Hijra would be