steady as a rock

steady as a rock idiom
Firm, dependable, as in Betty always knows her part; she's steady as a rock. This simile uses rock in the sense of “something that affords a sure support,” a usage dating from the early 1500s.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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