Solid as a rock - Idioms by The Free Dictionary
solid as a rock
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*solid as a rock
Cliché very solid; dependable. (*Also: as ~.) Jean has been lifting weights every day, and her arm muscles are solid as a rock. This company has always built power tools that are as solid as a rock.
(as) solid as a ˈrock extremely solid and reliable: The Irish team were solid as a rock in defence. ▶ ˌrock-ˈsolid adj.: The company has a rock-solid reputation.