mugger vs robber

mugger

noun
  • A person who assaults and robs others, especially in a public place. 

  • A large crocodile, Crocodylus palustris, of southwest Asia, having a very broad wrinkled snout. 

  • A person who makes exaggerated faces, as a performance; a gurner. 

robber

noun
  • A person who robs. 

  • An animal who robs. 

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