rock hard vs solid as a rock

rock hard

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

  • With strongly held beliefs and opinions, deep conviction. 

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

  • Having a very fully erect and stiff penis 

solid as a rock

adj
  • Extremely thick and heavy, so as to make it impossible to move. 

  • Very reliable and dependable 

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