footbag vs mud flap

footbag

noun
  • A small beanbag or sandbag used as a ball in a number of sports and games. It is typically controlled by the feet, but in some sports every part of the body except the hands and arms may be used. 

  • Any of the sports that use the footbag. 

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. 

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