Like a rock is an idiomatic phrase that refers to something or someone that is extremely strong, stable, or resilient. However, there are several synonyms that can be used instead of this phrase. For instance, rock-solid, unyielding, steadfast, unwavering, and resolute are all synonyms for like a rock. Each of these words conveys the solid and unshakeable nature of something or someone. Additionally, the phrase "like a mountain" is often considered a synonym for "like a rock" as both phrases suggest a sense of stability, strength, and firmness. Overall, there are numerous words and phrases in the English language that can be used instead of "like a rock".