What's the meaning behind ZQN designation for the zombies in I Am A Hero?
In chapter 220, it is explained that, officially, "ZQN" is an acronym made up by the US military that means "Zero Qualified Nucleus," though it is almost certainly a backronymic pun on the Japanese internet slang word "DQN" (pronounced dokyun):


  [ＤＱＮ]{ドキュン} (Noun)
  
  
  dumb-ass (slang, derogatory)
  delinquent; violent person; rough-looking person (slang, derogatory)
  
  
  
    -adapted from jisho.org