How did the term PAN PAN originate?
Wikipedia has an un-cited reference to the etymology being 


  As with mayday (from venez m'aider, "come help me"), the urgency
  signal pan-pan derives from French. In French, a panne ([pan], "pahn")
  is a breakdown, such as a mechanical failure. In English, it is also
  sometimes[vague] pronounced as /pæn/ ("pan").


This article supports that history (and has many of the other emergency phrase histories as well).