rock hard

Short definition from Wiktionary

Adjective

1. 1
   As tough, hard, or inflexible as a piece of rock.
2. 2
   With muscles developed and toned to a high degree, especially the muscles of the abdomen.
3. 3
   Having a very fully erect and stiff penis
4. 4
   With strongly held beliefs and opinions, deep conviction.
5. 5
   Alternative spelling of rock hard.