hard as a rock


Also found in: Wikipedia.

(as) hard as a rock

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

*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