Adjective
(simile) Extremely thick and heavy, so as to make it impossible to move.
We're not going to be able to break down this door: it's solid as a rock.
(simile, of a person) Very reliable and dependable
Stewart has been solid as a rock ever since becoming the new vice-chairman.
'Call Me' is not an exceptional Al Green album, but it is as solid as a rock at its center. Jon Landau