rock hard vs rock-hard

rock hard

adj
  • With strongly held beliefs and opinions, deep conviction. 

  • As tough, hard, or inflexible as a piece of rock. 

  • With muscles developed and toned to a high degree, especially the muscles of the abdomen. 

  • Having a very fully erect and stiff penis 

rock-hard

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