It seems highly unlikely that the Germans would have a slang term for the P-38, and not for any other aircraft, not the Spitfire, Mustang, Thunderbolt, Mosquito, Yak or Lavochkins.
I had always believed it to be a propaganda created name, just like "Whispering Death" and "Whistling Death" and Brooke's info in this thread has only reinforced that view.
I can't think of any dread inducing nicknames the USAAF, USN, RAF or RN created for the Bf109 or A6M. Perhaps "Butcher Bird" for the Fw190, but I honestly have no idea about the origin of that nickname. For all I know it could be the German nickname translated to English and no more relevant than Jug or Spit were to the Germans.