mud flap vs treasure chest

mud flap

noun
  • A flap that hangs down behind a vehicle's wheel so that mud and other things kicked up by the wheel will hit the flap instead of a vehicle behind. 

treasure chest

noun
  • Any source of something valuable or beautiful. 

  • A treasury, especially a state or royal treasury. 

  • A chest filled with treasure, especially one used by pirates, etc. 

How often have the words mud flap and treasure chest occurred in a corpus of books? (source: Google Ngram Viewer )