The term 'Solid as a Rock' is often used to describe something that is very STABLE and SOUND. This phrase can be represented in a crossword puzzle by two words: STABLE (6 letters) and SOUND (5 letters). STABLE means firmly fixed in place, not easily moved or disturbed, while SOUND means free from flaws or defects. Together, these two words accurately describe the phrase 'Solid as a Rock, ' which is used to refer to something that is reliable and dependable.