steady as a rock
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® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.