(as) hard as a rock

(as) hard as a rock

Very hard. I can't use this pillow, it's hard as a rock!
See also: hard, rock
Farlex Dictionary of Idioms.

*hard as a rock

 and *hard as stone
very hard. (*Also: as ∼.) This cake is as hard as a rock! I can't drive a nail into this wood. It's hard as stone.
See also: hard, rock
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • hard as a rock
  • (as) hard as stone
  • harder than the back of God's head
  • be hard at it
  • hard going
  • in the end
  • come by
  • honestly
  • hard to come by
  • hard to swallow