The term 'as a rock' is often used to describe something that is very strong and reliable. The two crossword solutions for this term are 'HARD' (4 letters) and 'SOLID' (5 letters). Both words refer to something that is strong and unyielding, and they are often used interchangeably in this context. Hardness and solidity are both qualities that are associated with the phrase 'as a rock, ' making these two words perfect solutions for this crossword puzzle.