punchbag vs punching bag

punchbag

noun
  • A person serving as an object of abuse. 

  • A tough bag, usually cylindrical, filled with air, sand, or other material, and suspended so that swings freely, which is punched or kicked when practising sports such as boxing or martial arts. 

punching bag

noun
  • A person serving as an object of abuse. 

  • A tough bag, usually cylindrical, filled with air, sand, or other material, often suspended so that it swings freely, which is punched or kicked when practising sports such as boxing or martial arts. 

How often have the words punchbag and punching bag occurred in a corpus of books? (source: Google Ngram Viewer )