hard as a rock
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(as) hard as a rock
Very hard. I can't use this pillow, it's hard as a rock!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
*hard as a rockand *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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.