Of some significance of late because of its use by the militant Jihadist group ISIS (Islamic State of Iraq and Syria) in Iraq. The nun stands in for “Nasara,” or Nazarene, and is at the time of this writing being used to mark the homes of Christians in Mosul. In a mimicry of the relatively tolerant practices* of the Ottoman Caliphate, Christians whose homes have been marked have been given an ultimatum to convert, pay a steep ‘tax,’ or flee.
The symbol has been taken up by Western Christians in solidarity with their Iraqi coreligionists.
*While certainly tolerant by 16th century standards, these strictures are clearly in violation of human rights in modern times.